FairWarning, while your intent:
I was referring more to the Mall Ninja variety than organized competition when I said you can't use 20.
was valid, the problem is that a lot of people will immediately assume "you have 12 magazines? Are you some kind of Mall Ninja?"
Typically those people are not serious shooters.
The people who say they have only one magazine probably don't practice much. (BlueTrain has made a point in many threads that he doesn't have much discretionary income for ammunition, so he practices very little.)
My point remains that there are valid reasons, which have nothing to do with Mall Ninjahood, for owning what some people would find to be excessive or at least redundant gear.