For my pistols I keep 4 or 5 for each one. I only have the two mags that came with my Kahr though. depending on how you look at it, I have ten mags for my carry gun (two OM 1911s, five for each, carry one at a time) and 20 or so for a GM (4 wislons, one chip ten rounder, and about fifteen el-cheapo GI mags from ammoman, two GMs, one works, the other is an AMT, lol).
I have ten mags for my AR15 and Thompson (plus the drum I never use). Two 20s and eight 30s for each. I keep one 20 empty at all times in case my other one decides it needs work (I cant close the door on my gunlocker with a 30 rounder in either one, the main reason I have 20 round mags).
Ive got my eye on some toolboxes that are going to be in the next employee sale. I know if I get any of them, I will prbably have two drawers full of mags, I cant fill them up yet (maybe one) but I'm not finished buying mags yet either.