I forgot you said these were just informal matches. 3 mags will probably be enough. I doubt they will have any stages needing more rounds.
Go with your selected pouch. You can always just flip the flaps over and tuck them in your belt or behind the mags. If you decide you like it, you can upgrade your gear later.
Go have fun. I think you'll find it's a lot more fun than just punching paper.
NRA member, DCF&S member, PAFOA member, USPSA member, NSCA member