For a centerfire auto how many mags would you be satisfied with at a minimum? Lets say if you shot 100 rounds a month from that pistol from a 9mm or .45.
I guess it depends on how you practice.
Loading mags at the range during a practice session gives me time to "think and consider" what's going on with my shooting. I have about 15 mags for my 1911's but rarely use more than two at the range during practice. 1911 mags are easy to load w/o a loader, plus I rarely shoot more than 5 rounds in a string.
If you're competing, then I'd say have two sets of how ever many mags you use during competition. In our local bowling pin matches for example, you bring 3 mags to the table, so for that, I'd say you should own 6 mags.