10 mags would hold 300 rounds.
30 can hold 900 rounds.
If I want to shoot 900 rounds in a class, I can load 30 mags and never reload once in the class...
Yes, I have more than 30 mags... SCARY!!
In the case of pistols, I have 12 mags for my Glock 17, they hold 17 rounds each. If I want to shoot 800 rounds in a weekend, I might have to do 3-4 reloads on each mag. Big deal. You do that in between instruction or while watching others shoot.