For me, it's a minimum of 10 for primary handgun(s) and rifle(s). Easy to accomplish with Glocks, the mags are dirt cheap. I buy 3 every year or so, and once I've run a couple cycles through them to make sure they function, they become "carry" mags. Other mags go into the range bag as "training" mags. I probably have 15-20 G17 & G19 magazines now. I have at least 15 Magpuls for my AR, and just pick more up when I find them cheap.
I'd like to get to the point of having 100 AR mags, just to piss off anti's, but eventually I'll run out of space in my safe...