Jesus Christ , I thought that maybe the Mongols would have twenty thousand, but on average they have fifty thousand. Try to get a coalition of nations behind you like the Holy Roman Empire, yes they may not like you but they probably won't like the mongols. So yeah try to get Europe to unite under a banner of some kind because if only you and the Byzentines try to take the mongols on, it will be too little to late. Form a grand alliance of europe and defend it from the mongols.