In every class I've attended, the lesson is clear: Shoot until the threat is neutralized. If one shot puts the threat on the ground, unable to continue the fight, so be it. If it takes two shots ... or three ... or four, that's how many it takes. One thing we discussed in one class was sitting in court while the prosecution asks, "Why did you shoot Mr. Jones nine times with your .357 Magnum?" Good question, and you might need a good answer.
"The gun has been called the great equalizer ... It insures that the people are the equal of their government whenever that government forgets that it is servant and not master of the governed." --Ronald Reagan