First, it's great that you are thinking about competition and USPSA in particular. Good for you!
Second thing, to answer your question: Round counts for single stages* can get up to 32 (or even higher in club or outlaw matches). You will often need/want to reload in between positions, even before the gun runs dry, and sometimes you'll want/need make-up shots. So you want
far more than 32 on your belt. I shoot in limited division, so I can have big magazines. Mine hold 19 or 20 rounds, and I walk to the line with 4. I never need more than 3...
unless I fumble one during a reload or get a jam and have to strip one out with almost all the rounds still in. Most guys shooting the restricted capacity divisions will have 5-6 mags ready on their belt, and sometimes will have an extra one to put in the gun at the make-ready command.
Third thing: It doesn't sound like you're currently on a path to having gear that is well-optimized to USPSA. I say this simply because, other than 1911's in 45 ACP, you're going to be at a disadvantage with 8-round mags, though it won't be crippling. I'll try to break this down:
- Production division allows 10 round magazines. So if you're in production, you'd be competing against other people with 10-rounders.
- Single Stack is limited to 8 round mags, but only for major, and that division is just for 1911's. If your gun is on the production list, it is necessarily not a Single Stack gun. So you're not going to be playing in that division. Nor are you going to be in Revolver if you're buying mags, which is the other place that 8-round guns are advantageous.
- Limited-10 is also limited to 10 rounds, not 8. Limited and Open and Carry Optics all have higher capacities.
- Although the rules say that a stage design cannot require more than 8 shots from one location/view, which is designed to make 8-shot 1911's never have to do a standing reload, it is rare for a match not to have opportunities to shoot more than 8 rounds. Plus, misses on steel are a thing (at least for me).
- In short, competing against guys with 10 rounds on board when you can only do 8 is going to be a bit of a disadvantage.
Now, what do you do with this? It depends. If you are pretty committed to your current gun, then buy enough mags to have a half dozen and get involved. If you are just starting to acquire gear, though, and think you care about having stuff that is reasonably optimized for competition, then you may want to step back and consider whether to devote money to mags for a gun that you may move away from fairly soon. Some people disagree with me, but I think there can be a lot of value in going to a match just to watch, get a feel for how it operates, and talk to the shooters about gear and other things. A visit to a match might make all of this easier for you to figure out.
Just to try to move this forward, though, what kind of gun are you contemplating using?
* Stages are like holes on a golf course, whereas the match is like the round of golf. Stages are the discrete, timed efforts/tests, and you can reload magazines between them as you progress through the whole match.