But, upon what do you base that number? Is it anything other than arbitrary?
If a gun is for defensive use, then it must be free of arbitrary guidelines like this. It is foolish to assume that you know how, where, when, and whom you will be fighting. If you knew things, you should just AVOID THE FIGHT. You don't know how many people you will be engaging, how many times you will miss because you are shaking to death, (even pros do this,) or how long you will have to wait for help.
Let me give you a real-life example. I live in Utah, which is a pretty peaceful state. whenever possible, I go out where I am unlikely to see anyone else, and ride my 4-wheeler. You know who else has this preference? Pot farmers. For several years now, Mexican cartels have been growing increasingly higher record-size fields of weed. Almost every one of these busts has happened in a place where I have hunted or explored at some time or other. The pot isn't the problem. The problem is, they baby-sit their grows, and because of the dollar value, and threats from cops and rival gangs, they are armed. So, here's the question. If I am out in the desert canyons, with an SKS across my handlebars, I stumble on something illegal, I immediately turn around and leave, and see the farmers in the road headed my way, how many bullets do you think I need now? Do you think that my 25-yard sidearm is enough?