In the late-twentieth century, an anthropologist and evolutionary psychologist named Robin Dunbar proposed a theory that Homo sapiens can only care about, identify with, and maintain stable relationships with 150 people.
https://en.wikipedia.org/wiki/Robin_Dunbar
Dunbar's number, a measurement of the "cognitive limit to the number of individuals with whom any one person can maintain stable relationships".
https://en.wikipedia.org/wiki/Dunbar%27s_number
By using the average human brain size and extrapolating from the results of primates, he proposed that humans can comfortably maintain 150 stable relationships.
