I go with an average of 10 per gun; although 3 per gun is my minimum for carry pistols, going with 10 allows for extended range sessions without having to waste time reloading.
Probably accumulated around 30 mags eacj for my SIG P-228, 229 and 226 pistols, since I have multiple pistols in these three platforms ... About 12 for my duty pistol (M&P) since I don't like loading mags during range sessions, as noted. 1911 mags ... gosh, a drawer full, maybe 100 or so?