Dunbar's number is a suggested cognitive limit to the number of people with whom one can maintain stable social relationships—relationships in which an individual knows who each person is and how each person relates to every other person

 "By using the average human brain size and extrapolating from the results of primates, he proposed that humans can comfortably maintain 150 stable relationships" - Wiki

Efter det gäller det att ha en stark gemensam historia/berättelse ...