Backwater, I know of no empirical data base on shots fired by private citizens in self-defense. I agree with you that since cops and citizens are armed against the same criminals, local police are a reasonable predictor of what might be needed. Most police, in NY state and elsewhere, carry fully loaded double stack service pistols. And in uniform, usually two spare magazines.
The current (February 2013) issue of American Rifleman, in its long standing Armed Citizen column, lists seven incidents. In each there was only one ARMED citizen taking action.
In only two of those cases was the Good Guy up against a single Bad Guy. In three cases, the armed citizen faced two to one odds. In the other two cases, there were FOUR criminals against the single armed citizen.
With only seven shots on the victims' side, the odds are not good.