I remember that Joel Spolsky said that for every 100k of yearly revenue, fogcreek will add another employee. And from my observation, this rule seems quite general among small software related business that is making real profit (all the startups that aren't making any money yet don't count).
One interesting corollary is that we can infer the revenue a company has by just looking at the head count of the company.
My question is, is there anyone who did a study on the relationship between the revenue and the company size? How valid is the 100k/employee rule?