I don't remember the source but I read about there being several inflection points in terms of team size where communication overhead changes suddenly...IIRC the first one is 8: beyond this you start needing to have 'managers' (their title may be different), i.e. people responsible for organization & team communication rather than directly making the product/sales/etc.
Anecdotally I noticed that even with 5 people you usually have someone starting to act as adhoc part-time manager, and this role quickly become too much for a part time responsibility as the team grows.
Is there some known principle that governs this, something that could be named along the lines of Parkinson's Law or the Peter Principle?