Hi, Max,

More would be convenient at times (so you don't have to be constantly reloading mags), but three are enough.

IDPA only allows you to have two mags on your belt and one in the gun at the start of a COF.

In that you need to start each COF fully loaded, you might stick one of your longer mags in your pocket to use as a "barney" mag (load one round from barney, then insert a fully-loaded mag).
Jane - Expert on nothing with opinions on everything...