Moshe continued, “A king must keep all the Mitzvos of the Torah! But he has three extra Mitzvos which help him remain humble and able to focus on leading his nation.”
Too many horses for a king is not proper. The Jewish king is not allowed to show off with his fancy-shmancy horses running ahead of him, as was the custom of all other kings (once upon a time… long ago, before the invention of fancy-shmancy limousines and ‘Air Force Ones.”)
The king is allowed to collect enough wealth to meet his needs, but, unlike the non-Jewish kings, he is absolutely forbidden to stock up on a vast amount of gold and silver. Too much money may lead to forgetting about the Torah and HaShem.
In those days, it was permitted for a man to have more than one wife. (Remember, Yaakov Avinu had four wives). Still, the king must focus on serving his people, without any distractions.