It depends on the caliber.
One hit of .45 ACP will ALWAYS be enough. It will cast men to the Earth.
One hit of .357 magnum... break out the steam cleaner as a body bag will not be needed. Instant vaporization.
But if its a 9mm? GOOD LUCK. They sell 33 round Glock 9mm magazines for a reason.
Honestly, if you a comfortable with even a 5 shot J-frame, have learned to load it in your sleep and quickly, and have confidence, it will do nicely in all but the most extraordinary situations imaginable. But that being said, if you can get good with a gun that has more capacity that is only a benefit, but it simply isn't the most important thing to me in a defensive handgun. I really like my Glock 19, but consider the capacity secondary to other reasons for trusting it so.
Shot placement, reliability, accuracy to some point, and above all the shooter's skill and confidence are much more important! JMHO.