First, apparently Merlin was not a part of the Middle Age.
The books on Merlin set the stories in the 'Dark Ages' around the 6th century. In the first few chapters of my current read, the Middle Ages proper only got started in the 11th century but since very little had changes socially in the previous 2 centuries, a lot of information dates back to the 800's.
OK, factoid #1: Around the turn of the Millenium, the population of England in estimated to have been roughly 2,5 million. 500 years later, towards the end of the Middle Ages, the population of England was essentially unchanged - estimated to be 2,5 million.