Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 18

לִכְתֹּב הַמֶּלֶךְ סֵפֶר תּוֹרָה לְעַצְמוֹ, יָתֵר עַל הָאֶחָד שֶׁלְּכָל אָדָם, עַד שֶׁיִּהְיֶה לוֹ שְׁתֵּי תּוֹרוֹת, שֶׁנֶּאֱמָר: "וְכָתַב לוֹ אֵת מִשְׁנֵה הַתּוֹרָה הַזֹּאת" (דברים יז, יח).

For the king to write a second Torah scroll for himself in addition to the one which [he is required to write to fulfill the mitzvah incumbent upon] every man. Thus, he will have two Torah scrolls, as [Deuteronomy 17:19] states: "And he shall write for himself a copy of this Torah."