I like to use Dunbar's Number (100-250) to approximate the levels of heirarchy in human organizations. The idea is that these organizations are most efficient when organizational layers don't exceed ~150 elements, due to the implementation details of the human brain.
Basically, you can do log_{150}(N) to get a very rough idea of how complex the organization of N people should be. This works for small startups and entire countries. Of course, startups should probably get comfortable with the idea of teams well before hitting >100 employees. Teams can then scale into departments (with new subteams), and once there are many departments, add regional layer, strategic/executive layer, and so on.
One interesting fact is that the USA population has roughly increased by a multiple of Dunbar's number since its organizational structure was codified in its Constitution. Perhaps time for another look?