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.

This number was first proposed in the 1990s by British anthropologist Robin Dunbar, who found a correlation between primate brain size and average social group size.

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

There is some evidence that brain structure predicts the number of friends one has, though causality remains to be seen.

Dunbar explained it as "the number of people you would not feel embarrassed about joining uninvited for a drink if you happened to bump into them in a bar.

Dunbar theorized that "this limit is a direct function of relative neocortex size, and that this, in turn, limits group size [...] the limit imposed by neocortical processing capacity is simply on the number of individuals with whom a stable inter-personal relationship can be maintained".

On the periphery, the number also includes past colleagues, such as high school friends, with whom a person would want to reacquaint himself or herself if they met again.

Proponents assert that numbers larger than this generally require more restrictive rules, laws, and enforced norms to maintain a stable, cohesive group. It has been proposed to lie between 100 and 250, with a commonly used value of 150.

Source

Dunbar is an anthropologist at the University College of London, who wrote a paper on Co-Evolution Of Neocortex Size, Group Size And Language In Humans

...ʇᴉ ʇǝƃ ʇuop noʎ ʇᴉ ʇǝƃ ʇuop noʎ ɟᴉ

50 sounds like the magic number

Human social networks might not be limited by Dunbars number — but Dunbar disagrees
A team of Swedish researchers has found no basis for the existence of Dunbars number, a hypothesized average of the number of individuals who can practically fit within a functioning social circle, often cited as 150 people.