Dunbar's Number is the proposed cognitive limit — commonly cited as roughly 150 — on how many stable social relationships one person can maintain at any given time.
The figure is named for British anthropologist Robin Dunbar, who in the early 1990s observed a correlation across primate species between neocortex size and typical group size, then extrapolated a predicted range for humans centered near 150. He supported it with historical and ethnographic group sizes: Neolithic farming villages, Roman army units, Hutterite settlements, and the headcount at which organizations tend to add formal hierarchy rather than rely on personal familiarity.
It is better read as a set of layers than a single constant. Dunbar describes concentric circles of decreasing intimacy — roughly five intimates, fifteen close friends, fifty friends, one hundred fifty stable relationships, and looser acquaintance tiers beyond that — each about three times the size of the one inside it.
The number is contested. Later work using larger and better-specified datasets has argued that the confidence interval around any such estimate is wide enough to make a single figure unusable, and critics note the primate extrapolation carries heavy assumptions. It nonetheless persists as a working heuristic in community design, organizational structure, and product decisions about group size — not least because social platforms expand the audience a person can reach far past 150 without expanding the number they actually keep up with.