My personal rule of thumb is that “enough” is whatever will definitely not run out before either I or whoever is trying to kill me is dead. Consider how long you could reasonably expect to remain in an armed confrontation, how much ammo you might expend during that confrontation, and how - logistically - that’s all going to work. Have a plan. That plan should include mags. Anyone trying to kill you isn’t going to hang out and sip tea while you reload mags. That plan should include backup firearms. The best firearms - perfectly maintained and fed the best ammo - can still jam. Anyone trying to kill you isn’t going to hang out and sip tea while you try to clear it. Pick up a spare and continue defending your life.
A couple of viewpoints:
1. "Mags," what are these "mags" you speak of? Belt fed is the only way to rock 'n roll.
2. Any ammunition you own that is NOT in magazines is NOT yours. It is still in the supply system.