Even if the drivers/users ratio is constant, raw #drivers does matter, given a fixed amount of space. As you get more drivers in the same amount of space, the minimum average distance from a user to a driver decreases.
Any driver that has a passenger is, for all intents and purposes, nonexistent when it comes to new passengers placing calls. You could have one million drivers in SF, but if 999,999 of them are on an active trip, and only 1 driver is free, then the fact that there's a million drivers makes no difference whatsoever.