The statistical average is 7 shots per intruder. There are tons of variables, how hopped up on drugs they are, how much cover he has, are you injured before you pull, can you not clearly see the target, how much adrenaline do they have, did they enter "fight" mode of "fight or flight" and 2 solid hits wont take them down, etc etc. Lots of variables that could reduce your accuracy or increase the enemies resistance. There have been many cases where 5-6 solid hits did not kill an attacker, I can recall one case of a woman trapped in her attic landed 6 shots with her revolver and the guy just walked up, left, and crashed into a tree a few blocks down.
Regardless, the statistical average is 7 shots per attacker/intruder. With 10 I feel like I have an okay chance, or at least a fighting chance, against two attackers. With less than that, my idea is it's a run away gun (Shoot if I have to, or shoot to clear the closest attacker and then run - to a bigger gun if it's in my home)