I think the law of common sense dictates you need at least as many bullets as it will take to render the attacker harmless to you.
As long as you feel you are still in danger from your assailant, you havent used enough bullets yet
Then you have to remember, stupidity breeds in numbers, so carry at least 2 to 3 times the amount of bullets you feel should have been safe to stop one zombie, cause they hang out in groups.
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Skeet is a sport where you are better to hit half of each bird then completely blast one and miss the other completely.
The choice is yours, place your faith in the court system and 12 of your peers, or carried away by 6 friends.
Nemo Me Impune Lacessit. 
'Nobody provokes me with impunity'
ΜOΛΩΝ ΛΑΒΕ
In this world there's two kinds of people, my friend. Those with loaded guns, and those who dig. You dig.
Clint Eastwood
The Good, The Bad and The Ugly