Congrats, and have a whole bunch of internets for correctly identifying either of the the artists who own the songs mentioned in today's title. Identify them both without Googling etc, and earn my respect in greater measure than at present. ;)
Anyway, what we're going to discuss today kids is a VERY basic framework (hence, the title) for how many Troops you want to take in a game at 1750 points (again, title.) None of these are hard-and-fast rules...as ever, if depends on some degree on your playstyle, and very heavily upon the rest of your list. Taking more Troops than recommended is something that can nearly be as bad as not taking any...let's overview.
If your army has insufficient numbers of Troops, it will struggle in one of the missions, no question.
Seize Ground becomes an uphill struggle for these lists (Tau a great example) as the enemy, if smart enough, will be able to focus fire down enough bodies to reduce your ability to mostly contesting. With Tau, of course, the things mobile enough to contest late-game are the things you've probably expended as blockers three turns before...Jet packs have a mobility that surprises many players, but it's not exactly optimal to advance T4 2W models on foot in the open very often - very contingent upon match-up and terrain. Also the present state of the enemy army. If they're reduced to a handful of huddled Gretchin on objectives, you get close and kick the shit out of them in CC if you have to, to get within 3". [Charging Grots with Fire Warriors would be hilarious...I've gotta see that!] This even applies when you can't harm something, btw. If a Land Raider is positioned slightly off an objective, you should be able to charge it and get models within 3" of the Objective below, with good placement. The fact that it won't be striking back means you can position them how you like with impunity - though this is best reserved for when there won't be another turn. Even if not though, it can force the opponent's hand, force them to do things they would rather not do...but this falls under another discussion topic altogether! :P
Back to Objectives...Some armies, for instance Marines, are very good indeed at taking, and most importantly, holding Objectives in midfield. Space Marines are really a midfield army, they exist to be driven forward, right up to the enemy, and then they just sit there, requiring the enemy to either have prodigious firepower to shift them from range, or to come and get them where they are - and with their own Damage oOutput increasing the closer the enemy gets.
To a lesser extent, the IG do the same, though IG rely more upon weight of numbers than the Marines here, who depend doubly upon their numbers and the inherent resilience of their Troops.
It gets said a lot, that ALL Infantry are fragile.
This is true. Even things like Deathwing Terminators can be torrented down without a LOT of hassle, with some above average rolling on your part, or below average on theirs. Very few units are as survivable as them though - and even when Wound Allocation is possible, when the models concerned are single-wound, the effectiveness is fairly limited. This, if you're interested, is why the Paladins and they're Apothecaries cost like 3e. A squad of Paladins can be wholly complex, if you wish - and, despite the loss of mobility, they are a LOT better than Nob Bikers too. Ah, off topic again...right. T4 models, aren't automagically survivable. Nowadays (thanks to Psybolt Ammo, Splinter weapons, and to a lesser extent, Eldar S6 spam) T5 is a much more reasonable baseline to expect a unit to weather a turn of fire. Even then, this has repercussions for T5 Troops too, as they go from being the uber-resilient models they were, to merely 'tough'...though they commonly have access to FnP, which doubles their survivability...sometimes more.
Essentially, this boils down to the fact that you cannot reasonably expect a unit of T4 Troops to survive very long in the open. In Cover, sure - that protects them from the worst of the incoming fire (eg, Long Fangs) and can protect from the less mobile units that may wish to target them (Leman Russes, Hydras, Dark Reapers [yeah, they're shit, but it's to prove a point] etc)
The ubiquitousness of Cover minimises the value of a 3+ Save, in ways that are rarely discussed in depth. When your save is 4+ in cover, and only 1 better in the open (and then negated by ALL the scary weapons) you have little incentive to pursue a path outside of cover. Doing so, is in many cases a trade-off - swapping your squad for one or more enemy units, with no genuine hope of survival...this is fine, when you don't Score. That's not to say it isn't EVER worth doing as a Scoring unit - Ravenwing and Melta Vets being the best examples I can think of of Suicide Troops Choices...but it's not ideal, and that makes it something you shouldn't aim to be reliant upon.
Ahem. Just typed another post, and now back to this.
Let's discuss Orks, briefly.
Orks have no save, essentially. Even in the very rare occasions when they DO, they pass even less often than Rokkit Launchas hit, so it's not worth taking into account. As T4 models, we have (hopefully) all agreed that that isn't nearly survivable enough outside of cover - even in Cover, it becomes dubious, not least because of Orks' propensity to die when any attention is focused on them in CC.
As a result, we can say that Ork Troops have a Low Survivability. Their Troops are designed (badly, Phil, for 5e...they were pretty damn good for 4e) to kill things, not to outlast the attention of the enemy. This is NOT what you want for a LOT of armies in 40k 5e - now, that generic statement may have seen your choler rise, but be calm. Tactical Marines? Designed to survive first, kill second. DE Warriors? To kill something, to get FnP. What is FnP if it isn't a Survivalist Mechanism? lol Tervigons? They are a MC with no real shooting capacity, and the ability to spit out their own Bubblewrap. Surviving first. Grey Hunters? They're cheap enough for a T4 3+ model with a typically decent Marine Transport to survive through numbers - even more so if on Foot. Death Company (as an example of a BAD unit of Troops) have survival mechanics built in [Relentless, FnP] but are wholly geared towards CC, something not good for a survival based unit...plus, of course, they have Rage, and don't Score. Space Marine Scouts - hardly any damage output, but often have a 2++ thanks to Stealth. I could go on. [Note that the GK Troops choices are much better balanced than others, in that there is almost an equivalent delineation between killing and surviving. This allows far greater tactical flexibility within the Codex - in ways, they have the best Troops Slot in the game.]
The way to get around issue like this are twofold - either you protect your fragile Troops by putting other things in the enemy's grill, and forcing the issue, or you simply have enough Troops to get around the issue. But, how many IS enough? Well, again, it depends hugely. BUT...
For Orks, I would suggest a bare minimum of 50 Boyz, if playing the Mike Brandt style of Mech Orks. Otherwise, any fewer than 80, I simply don't think will cut it...frankly, the GKs may have been the final nail in the coffin for any form of Foot Orks (Psychotropes are just DESIGNED to maul Orks) - it is too early to tell. Though, I said that Nids and DE would also finish off Orks except in the hands of exceptional players, and the Internet still thinks different, so...
For Nids - It's difficult. I figure it's somewhere around 60+ bodies, but I haven't played games with Nids enough to be an authority there...and GKs mess them up too. Certainly, I think Double T-Gon is great, but Triple would be a waste of points, even as HQ.
Eldar - the army about which I can speak most assuredly, and yet the army with greatest variation. :/
It depends entirely upon the type and number of Transports, but I usually have 20 scoring bodies in 4 units, and never feel it's enough. If I could get more in without compromising the list, though...well. That'd be something. Eldar get by better with so few due to the high resilience and mobility of their Wave Serpents, but it's VERY uphill nowadays. Basically, we're crap now, while pre-GKs we were distinctly mediocre and mono-buildy.
Dark Eldar - Again, a large degree of variety is to be found in the pointy-eared forces...mostly because Foot DarkDar is conceivable. Overall, I would suggest no fewer than 40 T3 bodies, I think. With Wracks, that changes quite a bit, probably 2 for 1 ratio, but 15 Wracks would be poor, even if 5 units of 3.
Space Marines - Thirty. Thirty T4 3+ bodies, because they are designed to be put right in the Killing Ground between forces. That said, it changes if they're T4(5). I'm thinking somewhere around 26 wounds in that circumstance.
Space Wolves - Again, I would aim for roughly 30, but could see 40-45 quite easily. That would hurt the ranged anti-tank though. If playing RazorWolves, I think 20-25 would be my aim, slightly fewer offset by greater firepower and more units.
Dark Angels - If playing Deathwing, around 20 may be sufficient. Ravenwing - fill all 6 slots. Gives you a minimum of 24 Scoring W, though I doubt you'd really have points for many more. Greenwing? - Get real.
Blood Angels - Depends hugely. Sang Guard are largely immune to Missile Launchers, so they cease to be scary in BloodWing/DanteWing/NippleWing/whatever you want to call it. Regular Jumpers want around 40+ bodies, Sang Guard probably 20 will scrape by, [unless playing against GKs :p] and if in Rhinos/Razors, the Standard Marine Bodies (25-35) should cover it.
Imperial Guard - My preference would be for 60+...but I can see as few as 40 working, contingent upon the amount of ranged anti-tank you have there.
Chaos? - Well, in a way, Plague Marines are better, even though they're worse. Basically, the Mono-Build won't change. 20-25 is what you want.
We can see a pattern emerge - if your Troops are T5, or have a 2+, then you can go roughly 10-15 fewer than otherwise. If both, that's a huge boost, and what rules are YOU using? lol
Number of Troops choices is also hugely critical in this - but, again, Marines kinda get a bye because of Combat Squads. I would say that any army that hasn't got 4+ Troops Units (excluding Transports) is fighting the rules as well as itself in SG missions, and having 6 only defaults to 'bad' when you buy bad or hugely overcosted units.
There, I think that covers it.
Remember, this is a Guideline to help players unused to the Game/Listbuilding/Competitive play, so it's deliberately vague, and glosses over a LOT of options and factors. It's impossible to do a comprehensive treatise on every build in the game, not least because new books will come out before you get nearly done and obsolete everything you've done.
The following armies were ignored, yes:
Tau, Witch Hunters, Black Templars, Necrons, Grey Knights, Chaos Daemons.
Necrons are crap, Tau are mono-build, Sisters aren't something I can comment on with enough authority 9and mono-buildy) GKs it's too early, and Daemons are something I prefer to defer to our good friend Brent. :P Overall, their T5 Troops are solidly survivable, but frustratingly useless at anything else. Horrors add an ability to kill things, even if it's not amazing - that lends you towards using 25-30, like Marines...only having a 5++ on Plaguebearers is a factor too.
Ciao for now!