20 million was probably around the low point of European population in the period between the fall of Rome in the 400's and the rise of Charlemagne around 800. At its peak, right before the Black Death wiped a lot of it out, the total medieval population of Europe was somewhere around 70 million. That would have been in the late 1200's, if I recall correctly. France alone held about 18 million in the 1280's. By way of comparison, India had a peak medieval population of about 200 million people just prior to the Muslim invasions (I don't remember which century that occured in), and it's a much smaller bit of land than the whole of Europe. China had about 125 million in the early 1300's, again right before the plague reduced the population significantly.

Another handy bit of info (the article waldronate linked to might mention this; I only skimmed it) is that on average, an acre of cultivated land grows enough food to feed 4-8 people. That may or may not be true, but it's an estimate that I've seen in a number of places, and one that I use in my own demographic estimates. And a single cow needs 2 to 3 acres of pasturage. Two animals to five acres. Horses require similar amounts of land, but they can graze in less hospitable country than cows.