My knowledge is that the rights to some albums end being packaged and sold in large batches, which can lead to some oddness when only some of an artist's back c

My knowledge is that the rights to some albums end being packaged and sold in large batches, which can lead to some oddness when only some of an artist's back catalogue is included - not that uncommon to see on Spotify, which also has albums appear and disappear as the rights change hands and deals get redone. So you can probably build a fairly large catalogue from a relatively small number of deals, I'm guessing it follows a rough 80/20 pattern.