Rashi on Deuteronomy 17:17/רש"י על דברים י״ז:י״ז

1ולא ירבה לו נשים NEITHER SHALL HE MULTIPLY WIVES TO HIMSELF only eighteen, for we find that David had six wives, and it was announced to him (by Nathan the prophet): “[Thus saith the Lord … I gave thy master’s wives into thy bosom] … and if that had been too little, I would add unto thee such and such as these (i.e. twice as many)” (Sanhedrin 21a).

2וכסף וזהב לא ירבה לו מאד NEITHER SHALL HE GREATLY MULTIPLY TO HIMSELF SILVER AND GOLD — but only as much as he needs for his soldiers’ pay (Sanhedrin 21b).