I forget the numbers I've heard, but that sounds right, It's also important to remember that in the 11th century, English vocabulary was much smaller than it is today, so those 10,000 words were a much larger proportion of the English language than might be apparent.

Another thing to know is that English was heavily influenced by Old Norse prior to the Norman Conquest, too. The mixing of those three languages, each having some differences in grammar and inflection, ended with English dropping a lot of inflections and turning to word position in a sentence to determine what's subject, object, verb.