הלכה על סנהדרין 41:27

From the laws of the commandments is that which they, may their memory be blessed, said (Sanhedrin 21a) that it is permitted to marry up until eighteen wives, but not more - and even if they are like Avigail, who was righteous, alacritous and good. [This] and the rest of its details are in the second chapter of Sanhedrin. And this is from the commandments which are only upon the king.