4 or 5 mag pouches with good magazines should suffice, plus a couple of external magnets so that on a table start you can grab a couple of magazines and slap them on as you run. Use a crappy magazine (often the one that came with the gun) as a Barney, bring it up to the line on a magnet, then at the line it goes in the gun, cycle, set safety/decock, drop that mag, and insert one of the good ones and holster. That possibly could have even been on a magnet too, come and think of it, so four reloads in pouches to use on the clock could suffice.
Anyone old enough to remember good old Barney Fife?
Don't weigh yourself down with all your mag pouches full on a stage that has a lot more running than round count.