The Torah in Parashat Shoftim (Debarim 17:17) says about the Jewish king, “Lo Yarbeh Lo Nashim” – he may not have a large number of wives. The oral tradition understood that a king is forbidden from taking more than eighteen wives.
The Sefer Ha’hinuch explains that a wife could potentially lure her husband toward improper conduct, and the more wives one has, the greater the risk of his being lured astray by one of his wives. The Torah therefore imposed a limit on the number of wives a king may have, in order to help ensure that he remains loyal to the Torah and is not led to wrongful conduct.
A king who marries a 19th wife transgresses this prohibition, and is liable to Malkut. The Sefer Ha’hinuch formulates this Halacha by speaking of a case of “Hosif Li’kah” – where a king “took” more wives than are allowed. The word “Li’kah” in the context of marriage generally refers specifically to Kiddushin (betrothal), and it thus appears from the Sefer Ha’hinuch’s formulation that the king transgresses this command when he betroths the 19th wife. The Rambam, however, implies that the king does not transgress this prohibition until he consummates the marriage with the 19th wife through intercourse. The Minhat Hinuch adds that according to all opinions, the king transgresses this prohibition anew each time has intercourse with the 19th wife, and so he would be liable to a separate set of Malkut for each marital relationship.