Unknown till after the attack is over this is unknown. Only one well placed bullet can end a single attackers attack, how many it will take to place one in the right place depends on situation and person making the shots!
That's the thing that cracks me up.
This illustrates the mentality of these gun grabbers. They say that we are such lousy marksmen/women that in a shootout situation in a public place, we will all miss the attacker and hit other people, children, dogs, and the trigger on a nuclear war head, but magically, in a home defense situation, we only need one bullet to stop a home invasion.
So, to answer the question of "how many bullets do you need" to stop a home invasion, my answer is MORE. Always more. Arbitrary magazine capacity limits, that MAY have a small probability of saving 2 people a year MAXIMUM, are not justified.