How many friends can a human brain maintain?

Robin Dunbar proposed in 1992 that humans can maintain about 150 stable relationships, extrapolating from the link between neocortex size and group size in primates. The number nests into layers of roughly 5, 15, 50 and 150. A 2021 reanalysis found the primate data too noisy to pin down a human group size at all, so treat 150 as an order of magnitude, not a hard limit.

Source: Dunbar (1992), Journal of Human Evolution; Lindenfors, Wartel & Lind (2021), Biology Letters