How many rounds are normally fired on one run through? The reason I ask is to figure out how many magazines I'd need.
This is almost an impossible question to answer as it varies immensely depending on the stages people decide to make. We've had stages as small as 8 rounds and stages as big as 40 rounds.
You will also, depending on division, likely reload before empty. This will, of course, effect your decision to bring more or less mags.
I'd bring as many mags as you have (up to 10) just to have them in case you need them. And to have extras in case one or two fail on you. Carrying a few extra mags you don't use isn't a large burden and you can always leave them in the car until you need them.
Another way of putting that: Any particular stage will likely not have any more required rounds than 40. (Normally the number is 32, but people are a little crazy every once in awhile.)
Now: That "required" part means "but you'll probably shoot more than this" if you add makeup shots for better hits, or need to make up missed shots on steel.
When I shoot a USPSA match, I normally walk up to the line with
at least 50 rounds available to me. Normally 60, and occasionally 70. (I almost never NEED them all. But I want them anyway!)
If this is your first match, you'll probably want to shoot in Limited division and have
at least three standard capacity magazines (meaning 17+ in a mag for 9mm--if you are shooting .40 or .45, going to need a minimum of more magazines!) that you can use on a stage, and also enough magazine pouches to carry them. If you have more mags, excellent. If less....don't miss!
After you've shot a couple of matches, you'll have a better idea of exactly what division you want to shoot, and what equipment you'll need for that division.
(For example: I shoot Production, and since I'm limited to only 10 rounds in a magazine after the start signal, I have to carry a LOT of magazines. In Limited division, you can load your magazines up all the way, so three mags is normally MORE than enough to shoot a stage.)