Complex sentence: Dunbar's number is a suggested cognitive limit to the number of people with whom one can maintain stable social relationships--relationships in which an individual knows who each person is and how each person relates to every other person.
Split into simpler sentences: Dunbar's number is a suggested cognitive limit to the number of people with whom one can maintain stable social relationships. These are relationships in which an individual knows who each person is and how each person relates to every other person.