Dora Bas Rivka Silver O'H
After the land of Israel is settled, the Jews will eventually want a king. (We saw this come to pass in I Samuel chapter 8.) God will select this king (as He did Saul and, later, David). The king must be one of the Jews, not a foreigner.
The king is bound by special rules. There's a limit to the number of horses he may have, because the people will return to Egypt to engage in the horse business. He may not have too many wives, because that leads to idolatry. Similarly, there's a limit to the amount of wealth he's allowed to accumulate. (See our Nach Yomi Companion commentary on I Kings chapter 10 to see how things turned out when Solomon thought he could outsmart the system.)
The king is required to write a Torah scroll and to carry it with him, to continuously remind him of God and His laws. This will ground the king and keep him from feeling superior to everybody else, as such egotism would lead to disaster. Following the rules would ensure a king's reign and succession.