Dunbar's Number
The suggested cognitive limit to the number of friends one can maintain, in terms of stable, social relationships, which is usually said to be around 150.
Origin
British anthropologist Robin Dunbar arrived at the figure through a 1992 study correlating neocortex size with average social group size across 38 primate genera. Extrapolated to human brain proportions, the regression predicted a natural community size of roughly 148 — conventionally rounded to 150. Dunbar extended the argument in a 1993 paper in Behavioral and Brain Sciences, proposing that language itself evolved as a form of "social grooming" to maintain larger networks than physical grooming alone could sustain. Malcolm Gladwell popularized the number in his 2000 book The Tipping Point, bringing it to a mainstream audience.