If you look through the history of the Armed Citizen column in the NRA magazines you'll see a lot of instances where the problem was solved without shots fired. I would venture to say that a good percentage, maybe even the majority, of Average Dude defensive gun uses would be just fine for the good guy without any ammo in the gun. That said, I don't think anyone here would advocate that as a plan.

My general rule of thumb is that I'll carry one spare mag, regardless of capacity. I want to know that if I get into a fight for my life I have the ability to top off my gun and have a full load. That way, I don't have to wonder about how much ammo I have on board or try to figure it out after an incredibly stressful event. If I go over one mag on the initial fight, well, I've really gotten myself into something big and I'll happily tap into the second mag at that point.

I've carried 3 full size spares as an experiment to see if I could pull it off, and I can, but it doesn't mean that I'm going to on a regular basis.