Minimum of 3 if it's a carry gun (1 in gun, 2 reloads).
For the rest, as many as seems practical. I like to have at least 2 per gun, but for the expensive ones, or those I seldom shoot, I'm ok with just 1.
There are a few guns I have been meaning to pick up a few more mags for...
“There are three reasons to own a gun. To protect yourself and your family, to hunt dangerous and delicious animals, and to keep the King of England out of your face.” - Krusty the Clown