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.
"If all guns were built with mechanisms that kept them from firing when held sideways, we could end gang violence." humorist Frank Fleming