I am really curious how many songs are recorded all these years which never ended up to an album and are stored in the vaults and will never see the light of day
I think easily 100+ songs since the Golden Heart sessions, and maybe that’s even a modest guess.
Yes, I think Mark has enough songs for a few posthumous releases of original material. That's an argument against the "Mark is an average songwriter" paradigm.
No, Mark is a genius songwriting machine, and the only reason he's so successful is because he always had enough songs to record since day 1. You take any other band, and usually, there's a struggle to assemble enough original material for albums. Even The Beatles recorded covers to fill the album until A Hard Day's Night.
With that said, I think some of these songs are "nomad songs", that appear in the subsequent albums, but I don't think it happens that often.