It is suggested that the number of neocortical neurons limits the organism's information-processing capacity and that this then limits the number of relationships that an individual can monitor simultaneously. When a group's size exceeds this limit, it becomes unstable and begins to fragment. This then places an upper limit on the size of groups which any given species can maintain as cohesive social units through time.
1 - The Individual 5 - Intimates 15 - Best Friends 50 - Good Friends 150 - Friends (Dunbar's Number) 500 - Acquaintances 1500 - Recognizable People
Dunbar, R. I. M. (1992). "Neocortex size as a constraint on group size in primates". Journal of Human Evolution. 22 (6): 469–493