The Estate has definitely more songs in their possession because right after the singer’s death, John McClain discovered around 60 unreleased, complete/nearly complete songs.
When they were compiling the final track list of the ‘Michael’ album, they chose from those 60 songs, as confirmed also by John Branca.
For the ‘BAD 25’ album that came out two years after, the Estate had in their possession an extra number of 60 unreleased, complete/nearly complete songs from the ‘BAD’ album recording sessions alone.
Also, L.A. Reid spoke about 24 unreleased tracks in a complete/nearly complete state from which they chose when they compiled the final track list of the ‘Xscape’ album.
The Estate and Sony Music also indicated that they aim at longevity when it comes to Michael Jackson’s future, posthumous album releases (that contain unreleased tracks).
This longevity (as their aim) means that all of Michael Jackson’s unreleased tracks (in a complete/nearly complete state of completion) should not get a release only in the following decade after his death, but they should get a gradual release on a long-term basis that covers many, following decades (as they stated, just like the strategy of Elvis Presley Estate).