Even 100.000.000 isnt all songs. There is still loads of music missing, with changes hppening every now and then as labels pulls specific band lineups not covered by their agreements with Spotify.

There is nothing by eg The Blues Brothers outside the two movies and their debut album. Nothing by Spocks Beard prior to Neal Morse leaving the band. Other albums are stripped of specific songs because a guest artist wasnt part of the overall deal.

There are probably countless other examples as well, and as artists are moving away from physical media completely I fear that entire artist will disappear in the future because they happened to be pulled from streaming services.

On the other hand, there are countless recordings of the same songs by covers bands, different recordings of the same classical pieces, etc. https://forgotify.com/ should give a good idea of what the long tail on Spotify is.