What you carry depends on the situation.
You and a partner both carrying, in a low crime area, out and about in the car, no spares, she has a revolver, I have a 9mm. That's 12-24 rounds. If we're going to the range we are both carrying 2 firearms, lots of spare mags.
Unknown situation, higher crime area, no partner, chance of being shot by return gunfire, then five is optimal. Two mags on the weak side, one mag on the strong side (reload if wounded), one in the firearm, one in the pocket for reloads after your firearm is empty. BUG with one extra mag (in case primary firearm malfx, broken spring, firing pin). You are basically being sure of having redundancy of both arm and ammo. The MAG is the weakest component for a semi-auto (it's just thin metal/plastic). You always want to presume you have one or more mags go bad and need to be dropped and replaced.
Always need 'one more' to reload after your firearm is empty. Outside in the world you never want to have an unloaded firearm (unless on purpose).
IMO, in many circumstances, having two of you armed together actually lessens the need to fire them. (tactics of intimidation...and laser dots, lol).
ANY mags in your pockets are suitable only for rearming your carry belt or reloading your firearm after a fight, and should not be considered 'action-ready'.
Having a partner carrying a revolver -can- obviate the need for a BUG in lower crime areas.
Finally, assume you will only hit 2 out of 5 shots in a real gunfight, and only 1 of those fatal. So divide the rounds you carry in half -at least- when you figure your needs. If you have a 19 round capacity, you will probably have 5-6 effective shots (aim deteriorating under stress). Most stats say LEOs and other pros hit 25-30%, iirc.
$.02