My quick answer over at PeerPong:
For someone like me, it's probably over several hundred since I enjoy more of the early stages of company development. One take is when an executive has to work more on people problems than product problems, or as an employee you have to be aware of bull-eyes that were never on your back before.
For someone that likes structure, it's probably over 5,000. Maybe 10,000? Depending on when too many layers are added and you get too many people managing upward. One example is my wife who went to Google from Morgan Stanley. At that time, Google was about 3,000 people and big enough to have a mature corporate structure but small enough to be entrepreneurial throughout most of the company. She's been there 6 years and still loves it, but after her first 2-3 years it became a different animal.