Who was the king of Cheshbon, mentioned in Bamidbar?
Perhaps you mean the king of Cheshbon?
Cheshbon was the capital city of Sichon, king of the Amorites ("Emorim"). The Jews conquered it.
See Numbers 21:21--27:
The verses indicate that the Amorites conquered land from the Moabites; some commentaries read these verses as saying that Cheshbon had always been Amorite ("let Sichon's city be built and established!"); others read it that Cheshbon was also part of the conquest ("let it become established as Sichon's city").