Charles
Five is a good number for mags to have but you only really need four pouches on your belt. what most people do is load a mag into the gun that is in the back pocket and rack a round into the gun. they remove the mag, safe the gun and holster it. they then put a round into the mag to top it back up to ten, then unholster the gun and insert the mag back into the gun giving them 11 rounds to start with. so that give you a total of 5 mags, one in the gun and four on your belt for a total of 51 rounds. the maximum stage in IPSC is 32 rounds so that gives you a safety factor of 60% (59.38 to be exact.)
Some of us carry 6 mags as instead of topping up mag #5 we just insert mag #6 into the gun and put #5 into our back pocket, however some do have the fifth pouch to store this short round mag into.