etc

So I'm doing a map on the side based on a comment I saw a while ago in the pre-1900 forum (which I can't seem to find, unfortunately) which basically made the point that had Thomas Aquinas lived longer and been able to complete various important texts then we might see social and technological...

With the presented borders below, theoretically how big population could the area of Kingdom of Hungary support in the Middle Ages? What are the biggest possible numbers? Thank you in advance for your answers!