“And he should not have an abundance of wives and his heart should not turn away!” (Devarim 17:17)
It is a negative commandment that the king should not have too many wives, and about this the Torah says “And he should not have an abundance of wives”. The reason for this commandment is explicit in the verse itself.
Amongst the laws of this commandment are that he may marry as many as eighteen wives but not more even if they are like Avigail who was especially righteous and swift to do good. This is one of the commandments which apply only to the king.
This commandment applies only at a time when we have a king. If he transgresses this commandment by marrying more than eighteen women, he receives lashes and must divorce the extra ones.