Depends on where you are I would guess, things like this will always vary by state and sometimes even by city. I don't THINK there are limits on amount of ammo you can hold anywhere, but I know there was a proposed amendment here as part of this (https://en.wikipedia.org/wiki/NY_SAFE_Act) that would place limits on the amount of ammo you could buy within a certain timeframe, don't think it passed though. And then you have someplace like Texas where you can basically do whatever the hell you want.
But 100 rounds you go blow through in no time, that's just a fun afternoon target shooting.