How many bullets does one need for self defense?

The argument I've heard is funds are needed to defend oneself or one's home.

I can't think of a time that I heard of anyone having to shoot more than a few shots in such a situation.

Does anyone really need more than ten rounds in a magazine?

Edited for the answer: Only a few. If you can't stop someone with a few rounds, you're either likely to hit innocent people with you 83% of misses (based on replies to the thread), or you won't stop the evil-doer before it (zombie, in this case) gets you. If a horde comes after you, it won't make a difference if you had 5 shots or 20; if they want you bad enough to come in high numbers, they'll get you.