Very, very good advice from JD. I carry extra mags (and a "high" capacity pistol), but I don't consider them as an absolute part of my ammo. I plan on only having what's in the gun - if I am able to reload, that's a bonus.
I have big hands, and the double-stack .45s still seem pretty thick to me. I've held a few in other calibers that seemed a little thinner, but not all that much. If you can manipulate them well, carry the weight, conceal them appropriately, and they are RELIABLE, there's no reason not to have more ammo on board.