The 363rd prohibition is that a king is forbidden from having too many horses.

The source of this prohibition is G‑d's statement1 (exalted be He), "He must not have too many horses."

The limit is that he may not have even one horse for the purpose of running before him.2 He may own only the animal that he himself rides upon, and horses kept ready in his stables for his army to ride on in battle. For himself, he may own only one single animal.

The details of this mitzvah are explained in the second chapter of tractate Sanhedrin.3