A typical 3' x 6' table holds about 200 hardcover or trade paperbacks; an 8' table holds about 250. The same tables hold about 350 or 450 mass market paperbacks, respectively. Of course, you probably also have several boxes on the floor from which you replenish the tables.

You don't need to be exact, but there is a big difference between 2,000 and 10,000, so try to come up with a reasonable number - even though you will probably feel like you've sorted and carried 50,000 books!