Guessing that you're just starting out...
Most of the skirmish game sets are 20min-1hr long around here. You'll certainly have enough ammo
Most new guys would just take one or two hicaps at the most for skirmish games....just fill it up from a BB bag inbetween games. If I had to guess I'd say that most guys take 8-12 mid cap mags (avg 10 mag @ 120rnds each = 1200). It's completely dependant on who much you shoot...some might not get through a single mag....others might end up emptying them all.
While it's completely understandable to want to just blast the shit out of everything when you first start....try getting some mid cap mags. Having to change mags and judiciously use your ammo really adds to the excitement of things.
Best of luck