How many mags do you need?

I am finding that I prefer to have enough mags with my pistols to shoot a standard box (for whatever a standard box is, I think 50 rounds) of ammo without reloading mags. 50 rounds also gets me through pistol qualifications. Therefore my single stacks require a higher mag count on hand than my double stacks.

My AR has about 10 mags in its case. I figure that will get me through the first wave of whatever I need it for.

So what's your rule of thumb on mags?

How many mags do you need?