Do you mean the Studio Classics line, or the numbered special editions (Planet of the Apes, etc.)? Studio Classics goes up to 40 (The River's Edge), but has been supplanted by the Cinema Classics line. The special editions were numbered on a seemingly random basis.
33In Old Chicago1937Henry King 34The Man in the Gray Flannel Suit1956Nunnally Johnson 35Orchestra Wives1942Archie Mayo 36The Rains Came1939Clarence Brown 37Two for the Road1967Stanley Donen 38The Black Swan1942Henry King 39The Keys of the Kingdom1944John M. Stahl 40The River's Edge1957Allan Dwan
Each of these DVDs has a number present on the spine