Dunbar’s Number
You learn something new everyday. Apparently, there is a theoretical limit to
the maximum number of individuals with whom a set of people can maintain a social relationship, the kind of relationship that goes with knowing who each person is and how each person relates socially to every other person
and that number is called Dunbar’s Number. The number is a significant value in sociology and anthropology.
There is a very interesting analysis on the Dunbar number in massively multiplayer games in this article – “The Dunbar Number as a Limit to Group Sizes“.