It is not about how many it takes to take out one assailant, it is how many assailants you have to take out.
Kinda off topic for the funny picture thread, but I'm actually a bit worried about how many rounds it takes to take out one assailant. With the rise of these cheap, synthetic drugs coming from China, there's been several stories were a cop had to do a mag dump or two just to get one druggie to calm down. The one that really sticks in my mind was the guy in Florida that was eating some other guys face while he was running around naked.
I've always got at least one extra mag on me as well.