I asked a similar question a few weeks ago about the total number of records that could be played and accepted by the average Northern Soul night crowd.
One of my points was that as an example, John Manship lists about 20,000 NM tracks and another 6000 xover etc in his price list book. (Of which I only know about 40%)
Then you have some good b sides, lp tracks, tracks after 1974, unlisted tracks like Willie Brown Get out and get it, never mind the unreleased motown and other label tapes.
I read that there is only an estimated 7000 tracks ever been played at 'All Nighters'. So in answer to your original question, assuming the DJ's are brave enough to play something that may clear the floor, there could be +30,000 tracks that have never been played or if they have only a small number of followers have ever heard of them. (IMO)
Ed