The number of people with whom one can hold personal relationships is claimed to be limited to about 150 individuals. This is known as Dunbar's number after Professor Robin Dunbar, head of Oxford University's Institute of Social and Cultural Anthropology. This limit is set by the size of human brains, and fits a general pattern relating brain size and social group size across the monkeys and apes. Relationships survive only if one reinforces them by occasional face-to-face contacts.
Circles of acquaintanceship include one's biological relatives as well as more casual acquaintances, but it does define the people with whom one has reciprocal relationships of trust and obligation. It demarcates those whom you know as individuals from those whom you recognise but only have casual relationships with.
This circle of 150 is not an homogenous social group: it consists of four layers, the circles of acquaintanceship, which scale relative to each other by a factor of three (an inner core of five intimates, and then successive layers at 15, 50 and 150).
With each successive circle, the number of people included increases but the emotional intimacy decreases.