Malcom Gladwell in his fascinating, albeit simplistic, book The Tipping Point. Introduced to popular consciousness the idea of Dunbar’s Number (which is 150). The significance of Dunbar’s Number in the mind of Gladwell and many anthropologists is immense. Dunbar while researching primates discovered a correlation between relative sizes of a species’ neocortex and the quantity of individuals within their social sphere. The social sphere being the network of individuals with whom you share direct personal relationships. In the case of primates that was usually members whom shared resources and would not attack outside of particular incidents.
With some extrapolation, Dunbar asserted that 150 was the maximum number of individuals with whom a human can maintain sustainable and meaningful interpersonal relationships. Taken at face value Dunbar’s claims are simple enough to apply to tribal villages or even military units, as Gladwell describes. However, the caveat Dunbar makes is that humanity has a “cheap” method of maintaining relationships - language. This is neither time intensive nor particularly binding as far as relationships.
Primates cooperate to fulfill a mutually valued purpose and to sustain each others survival. Interpersonal relationships are necessary to maintain at a close degree because communication amongst primates who are unfamiliar with each other is cumbersome and faulty - not something that is useful when a leopard is on its way. Humanity allows for communication without the relationship - via language. Thus Dunbar’s number takes this idea of “cheap” social sphere into effect.
That said, anecdotally it would appear that 150 is the upper-bound rather the mean. If one examines effective company cultures, online communities, schools, prisons, parishes and the like one finds that 150 is unusually large. Groups that large often suffer from the tragedy of the commons, wherein individual goals and collective benefit are either incompatible or contradictory.