Dunbar's Number is a theoretical cognitive limit to the number of stable social relationships a person can maintain, typically estimated to be around 150. This concept was proposed by British anthropologist Robin Dunbar, who suggested that our brains can only handle a certain amount of social connections effectively. The number is based on the size of the human brain and the social structures of primates. It implies that while we can have many acquaintances, only a limited number of people can be considered close friends or family, impacting how we interact in social networks and communities.