How would Humans be the most numerous race?

[Excession]

Humans build cities. Real cities. The high elves may build their shining towers in the forests of faerie, but they house no more than a few tens of thousands. And the woods elves build only pale imitations of that. The dwarves amass only as many as they need to mine the current resource, then move on to the next. The Drow come close, but a slow motion disaster isn't a real city.

Humans put a million people in one place, and somehow manage to thrive without going mad with the noise and crowds. As it goes, quantity has a quality all it's own. It's hard to kill a million people, harder still to compete with them. Even if you do, they have so *many* cities, and seem to keep making more.