Mongols were a very dangerous foe... I believe the only real conflict between Europe and Mongols was in Poland... and the European knights got massacred. The mongols just killed the knights mount and he was utterly screwed once on foot. Even if his horse was very well armored, they would be slow and tire quickly and could never hope to catch mongol horse archers.
So in reality I don't think even 1 knights could kill 1 mongol horse archer. Sure if the mongols sent their lancers they would get slaughtered... but army for army the knights would have little tactical importance against the mongols.
They were also exceptional foragers and brilliant at laying sieges and using scare tactics. They also kept great order, and there empire only really fractured because of rulers. It was through no lack of their military that they fell, and I can't think of any army that actually killed MORE mongols than they lost men.