Haidt doesn't have a fixed number in mind. He started with Richard Shweder's list of three moral foundations which seem to have a firm grounding in psychology, evolutionary biology, and anthropology, and then went looking for more. At one point he even offered prize money to people who suggested a promising new foundation. The sixth foundation that he added, liberty/oppression, was based on the suggestion of a prize winner (the psychologist John Jost, who has his own theory of political psychology and has been one of Haidt's harsher critics).