I'd have to say 10 minimum total. Whether that's 5 + a reload, or 7 + a reload, or 10 in a single mag... 10 seems to give me the feeling of comfort to take on any reasonably foreseeable situation. 1-3 perps, a few shots at each.
Minimum caliber .380. Preferable caliber ranges 9mm/40/45.
Of course, I want more than 10, so I generally have 15-30 rounds, which makes me feel better.
And there are variables, that include time of day, where I'm headed, how far from home, etc. Further from home/deeper from an armed police response means that I want more ammo/guns with me. Out of state trips nearly always include 2 handguns, a long gun, and lots of ammo.