There are many different methods taught on this, such as double tap, two to the body and one to the head, etc... I was taught by my father, who was a police firearms instructor in the days of the revolver, to fire 3 center mass and check for a threat. If needed, 3 more then reload (on a revolver of course). I still practice the 3 and assess rule and feel good about it. I carry an XD 45 that has a higher capacity, but 3 is an effective number and should not damage you in court (unfortunately always a consideration) for excessive. The key to any of the methods is to PRACTICE. They have to be second nature. I always spend range time drawing and firing 3. It should be instinctive, whether it is a double tap or a triple.