The number of rounds the mag holds determines the number of that mag you should have. EXAMPLE: 10 round mag = 10 mags owned, 30 round mag = 30 mags owned. Then, multiply by the total number of firearms you own, and divide by two. If you have any cash left, get a few more of each. Of course, anytime you find a real steal, you should add more. And don't forget to factor in your current age.
One day our kids, or your grandkids may be glad you did! Nothing sadder, or more useless, than firearms with no mags.