Caliber is secondary to being able to hit your target. There's the old saying, "A hit with a .22 is better than a miss with a .44."
I routinely carry .380, .38 Special, 9mm, and .45 ACP. The only time I feel "under gunned" is when I have nothing.
When it comes to 9mm v. 40 v. 45 etc, when it comes down to it, there isn't much difference in terminal performance when comparing modern defensive loads. Wound cavities are very similar in addition to penetration depth.
As far as capacities go, I don't know of anybody emerging from a gun fight and then complaining about having had too much ammo.
There is no such thing as "knock down" power in handgun rounds. What is important is having penetration to reach vital organs. For the smaller calibers this may require sticking with non-hollow point or ball ammo.
Most people die from handgun wounds because they have predisposed themselves to dying. Short of a strike to both lungs, the heart, major artery, or the central nervous system, most handgun wounds are survivable given what modern medicine and proper first aid is capable. Hence a saying I've picked up, "If you're shot you're only hit. If you're dead you don't know it."
Another problem is people in altered mental states. Your tweakers and dusters that don't feel pain couldn't care less that you just put a .44 Mag round straight through their heart and left it in ribbons. They've still got upwards of 30 seconds of functioning left before the oxygen in the brain is depleted and they ultimately expire. A lot can happen in 30 seconds, especially if the subject is armed.
The first rule of a gun fight is to have a gun. The first rule of a knife fight is to have a gun.