Assuming that most of William's children had at least ten children, and keeping in mind that because these were wealthy people most of whose children tended to survive infancy, William and Matilda are ancestors to probably millions of people today. One estimate is that between 1 and 1.5 million Americans alone can claim descent.
Assuming that individuals in each generation produce an average of 5 children every 35 years, until 2005, that would allow 27 generations from 1066 to 1915 leaving 10*5^27 descendants, or approximately billions of BILLIONS of descendants.. Assuming people never married their distant cousins (which they do). Neverthless, it would be kind of surprising if many people of European ethnic origin aren't descended from Wiliam the Conqueror.