Dunbar Number: Rule of 150

Wikipedia: "The so-called rule of 150, states that the size of a genuine social network is limited to about 150 members (sometimes called the Dunbar Number). The rule arises from cross-cultural studies in sociology and especially anthropology of the maximum size of a village (in modern parlance most reasonably understood as an ecovillage). It is theorized in evolutionary psychology that the number may be some kind of limit of average human ability to recognize members and track emotional facts about all members of a group. However, it may be due to economics and the need to track 'free riders', as larger groups tend to more freely allow cheats and liars to prosper."