1 should be enough if you hit them between the eyes...or through the heart...or snap their spine...etc.
Now time...that is another matter....I wrote this down from some article I read a while ago; it smacks of veracity:
"Time: The time you take to position yourself on the range is probably longer than most gunfights last. Confrontations are sudden affairs, over in 2 to 3 seconds, in the majority of cases. Almost never is there time to cock your gun, stand in profile, and take careful aim at your target. You and the suspect together are not likely to fire more than 3 shots, whether the situation is an ambush, armed robbery, or family fight. In the majority of cases, the initial exchange of fire will determine the outcome. The speed in which you can react to danger, therefore, is likely to be a more critical commodity than the amount of ammunition you have at your disposal. You probably carry more ammunition than you will ever need, but time is almost always in short supply."