I say design the pistol to fit the hand and the task, and then see how many rounds fit. My Ruger holds 15, and that's great. A 1911 holds 8, and that's great too.
Soldiers and cops have jobs that involve prolonged firefights, and they need all the capacity they can carry. Most civilians in "social encounters" fire only 2 - 3 shots. There's no way to predict whether you'll need more, but statistics says that the likelihood of need decreases with each additional round (4 is highly likely, 5 a bit less so, and by the time you get to 15 - 20, it's way out on the thin end of the bell curve).
If you carry almost any modern handgun and a reload (mag or speedloader), you should be stocked up for 99.99999999% of whatever you might encounter.
NOTE: This is not to say I support the stupid 10 round rule, because I don't.