I shot four or five matches with 7 and 8 round magazines, and this was back before Limited-10, so I was shooting a 1911 single-stack .45 with these in LIMITED class. (I didn't win...) It's good practice to change magazines anyway, right? Plus you have to plan where you're going to make the changes. But you have to work on planning the stage anyway, this isn't something that comes naturally, so this isn't a bad thing.
About the time I surmised I was likely to continue the sport, they came out with Limited-10 class.
In the intervening years, I have since accumulated some 20 or so 10-round magazines, of various brands, and also bought a CR-Speed holster, SafariLand velcro belt, and Uncle Mike plastic mag carriers. I never start a stage with less than 6 10-round mags of exactly the same brand on my belt, and keep a "charging" magazine of 8-rounds in my back pocket.
Now the USPSA announced a Single-Stack class with 8-round mags as the maximum. If you want to save a little money, stick with good quality 8-round mags, minimum six, preferably identical. (But discreetly number them, for reasons that will become apparent over time...) These mags can double for both competition and carry if you do it this way. That's the only reason I suggest it. Idea is you can train with what you carry. Since we can't carry here, that's only a fantasy for me, so I don't know. If you don't care about these things, start on the 10-rounders, but better, seems to me, if all are the same brand. Kind of messes with your head to have a mix of some 8 and some 10. If you're going for 10 better to get a bunch at once, even though costly.
You can get a good, minimal, fast-releasing holster, like the LimCat, or the CR-Speed, or many other various 'race' holsters. (As far as I know there are no holster restrictions in SingleStack class, please correct me, somebody, if this is incorrect.) This mainly enhances the cool factor, gives you something to spend money on, and makes you feel faster on the stage (in reality time difference may not be significant.) Downside is, you're not practicing with what you carry with.
Shoot safe!