It depends on the match, and the club you are shooting at.
I've shot at clubs that have three stages, sometimes four; and others that always have six. Or, anywhere from 50 rds to 150.
The match director should give you some rough idea if you can contact him/her.
The two places I regularly shoot at will email everyone on the email list a couple of days ahead and give an approximate round count.
BTW- I usually take 2X what they call for, just in case.
I said all that to cover me for this:
I would guess 100 rounds would do for most matches.
If I were going to a match for the first time somewhere, I would probably be very comfortable taking 200 rounds. That would cover my usual 2X safety margin at most matches I've ever been to.
But if you didn't know, and only have 100 rounds on hand, I would say go anyway.
I plan our club matches so that 100 rounds (a "Value Box" at Wal-mart) will get you through. Ammo has gone too high, and in some cases gotten to hard to find, to have stages that cause you to have to blaze away just to get the round count higher. That said, I try to set it up so that around 75 rounds are required on 6-8 stages. Some people miss, some more than others:biglaugh:, and have to shoot more than once so the actual round count is usually around 100.