I buy spare mags whenever I find a good pirce, or even if I just stop by the gun shop (I always make a point of spending about $20-30 per visit. It lets me fondle all the firearms I want, and sets a good standard for haggling when I buy a gun--the owner knows I spend money in his shop). I really couldn't give an accurate count of how many mags I have. I can guarantee that I have at least 2 per; up to 20, 30+ when it comes to AR's and G3 mags. heck, I was cleaning out the closet the other day and found a bag, I think from a gunshow last year, that had 3 Glock 17 mags. Completely forgot about 'em.
Really--you can't have too many.