It was lawful for the King, and for him only, to marry his predecessor’s wives. That David actually married any of Saul’s wives does not appear. Only one wife and one concubine of Saul’s are mentioned. See 1 Samuel 14:50 & 2 Samuel 3:7.
RE: It was lawful for the King, and for him only, to marry his predecessor’s wives.
Well, there were non-Kings who had more than one wife.
Just to name two...
Jacob had two wives - Leah and Rachel.
Elkanah had two wives, Penninah and Hannah ( who was the mother of the prophet Samuel ).