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!