I read somewhere that in the average shoot out involving police, the officers involved fired more than 30 rounds each. I don't remember the source so I could be mistaken, but in the average crisis situation in which you are forced to react with lethal force, I would assume you would need to react similarly. Now you have to take into account that you are not a police officer, and have no obligation to bring the criminal to justice. You are only supposed to protect life and your residence from a threat. So if you fire one shot and your assailant heads for cover, get the hell out of there if you have the opportunity. If you need to stay and fight to protect yourself or others, be prepared to handle the situation as best as you can. Training and practice are worth the thousands of bullets, if you train and prepare properly.