Full load in the weapon and at least 1 spare mag, I usually try to carry 2 spares though. It comes back to sun tzu's principals of thirds, for some reason that always stuck with me. And I read an article somewhere I think by Mas Ayoob advocating the same thing. 1/3rd for the initial assualt, 1/3rd for the follow up and 1/3rd in reserve.
So it depends on what I am carrying if I have the .45 then I carry 24 rounds. If I am carrying the 9mm I have 48 rounds.
A fully loaded firearm and a spare mag. The average firefight lasts 7 seconds. I figure if I can empty one magazine, reload and empty another in that 7 seconds, I'm doing pretty good. I usually carry a small BUG too.