It's exophoric referencing. When the ''one'' is already understood. So the person just says ''Have a good one.'' In this sentence one is used a pronoun.
Anaphoric reference: John is tall, funny, smart and he likes to play chess.
Cataphoric reference: He is tall, he is funny, he is smart and John loves to play chess.
Exophoric reference: He is tall, funny, smart and he likes to play Chess.