He means "scale the number of people working on the project at one time."

He's basically saying that if you have 10 files and 2 developers the chances of collision (or merging issues) is small, but having 20 developers working from 10 files makes things more sticky.

To some degree, he's not incorrect, in that over time, you'll spend more time managing the merging and conflict process than you will writing code.

Obviously, there are modern tools that will help alleviate this problem, but the deeper you go, the worse it is. This was the reason that "old" VCS had exclusive locks.