So basically ca. 1,000 in total (do those numbers include SORN?)
The database covers all vehicles that have a valid tax disc or a Statutory Off Road Notification (SORN). It doesn't cover cars that were off the road prior to the introduction of SORN, or cars that have never been registered on the road since manufacture.
So a car like that unregistered, yellow 16v turbo with only 825 miles, that kept popping up for sale, probably wouldn't be on the list (if it's still unregistered, of course).