You don't know WHAT you might need to defend yourself -- whether your bullets will penetrate, whether you will have enough ammo, will reload fast enough, or will be accurate enough fast enough. Hopefully, whatever you bring to the fight will be enough. A wise man would bring the very most suitable weapon, and the highest level of skill he can muster, and then pray that his best (tools and skills) is enough.
No one ever, ever, ever said, "shucks, I wish I had my cool old engraved, case hardened, beautiful, classic six-shooter that day... that Glock I had to face those muggers with had too much ammo in it, and I was able to shoot it way too many times."
Will you NEED 15 rounds and a fast reload? Who knows? Will you NEED to be able to shoot 0.15s splits and a sub 1-sec "failure drill"? You can't say.
Maybe you will pull our your Bisley and drill him through the forehead. Maybe he'll see it and run away! That's all great!
Or maybe you'll empty it into him (or into the background) and then be killed by his partner, while trying to shuck your empties. You don't know, but everyone's got to decide that they're willing to gamble that whatever they've got is enough, and be at peace with the "what ifs."
Defending yourself is ALWAYS a matter of being "good enough" or not. You don't get to control a lot of things about what will happen to you. Enough gun and enough ammo is one of the few things you can directly influence.