MDS,
Assume your talking about concealed carry? If so, one full mag should be plenty for self defense. If you think it could turn out like the O K Corral, then you better buy some double mag holders for your belt!
Whether you should may depend on your primary and if you have a BUG. I'll go for >10, and if that means carrying an extra mag, so be it. If not, I'm comfortable. Generally I'm 6+1 with an extra mag.
They may or may not if you wear them on your belt. Depends how you dress and what gear you have. I used to wear a mag or two on the belt but I have so much other stuff on my belt that it makes more sense to keep a mag in my weakside front pants pocket with nothing else. Less motion than reaching up to the beltline too. (Clean pocketlint often)