Pretty much what others have said.
5 bare minimum.
Carry/stash mags kept with carry ammo and fired occasionally for function and kept clean.
5 more for training/dumping on ground etc.
5 more kept new to replace mags that may get lost, damaged, or worn.
So 15 for me just starts to get comfotable. If your concerned about inheritance, discontinued production, law restrictions, trade fodder and/or profit after law restrictions, you'll want more than that.
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