Consider a 32 round stage with the following shooting positions:
Position 1) 6 rounds
Position 2) 6 rounds
Position 3) 6 rounds
Position 4) 6 rounds
Position 5) 8 rounds
To avoid a standing reload at any of these positions, you would need to have one mag in your gun and four on your belt. I like to go with five on my belt (and one in the gun) in case I have a malfunction.