@AlwaysThere, these 24 songs were the vocally completed songs.
Check also what L.A. Reid explained on TODAY.
"
As Reid explained on TODAY Tuesday, he had hundreds of unreleased songs to choose from [for the 'Xscape' album]. He said he looked for vocals 'that had great melodies that would still stand up today', adding that he was more likely to select songs with multiple takes from Jackson" (Randee Dawn, TODAY, 2014)
After all, they combed through 4 decades of Michael Jackson's vaults, so discovering hundreds of vocally incomplete, unreleased songs should not come as a surprise.