Think about it,
The reason we carry, is because bad things can happen. If one bad thing can happen, (you get attacked by a bad guy,) another bad thing can happen. (Your pistol can jam.) Admittedly, this is mitigated if you carry a revolver, but it seems a little limited to assume that ONLY the things you think about can go wrong. Besides a weapon, you should also be reasonably fit, have a cell phone, wear shoes you can run and fight in, and a good folding knife is probably a good idea too.
The main reason to carry a reload is to clear a malfunction. Higher capacity is less important. If you have a job that's too big to be done with ten rounds, you should be using something bigger than a pistol anyway.
I always carry two spare 1911 mags, for a total of 25 rounds for my Kimber. However, recently I have been considering switching out one mag for a good flashlight. I think it is much more likely that I will need the light than a third mag.