Realistically 10 won't be allowed to leave, purely because it'd be a nightmare to adequately replace them as you said (although the likes of Jones and Mata hardly need accounting for..). But as you also said, Lingard, Pogba, Mata, and Cavani are gone, so that's already 4. Ronaldo is a dead cert to leave, surely? He isn't going to want to be part of a complete rebuild in which we aren't challenging for trophies. Pereira is going to be sold, Henderson will be desperate to leave and will push for it, Jones and Bailly are surely off. That's 9 players that I'd say are extremely like to leave or are guaranteed to leave. Martial and Donny probably want to leave, but it won't be sanctioned by virtue of the fact that we need the numbers (although Donny might want to stay to try and prove himself to the new manager - if it's Ten Hag he'll obviously stay).
It's insane and such a mass exodus feels quite unprecedented. I think we'll sign about 4 and bring the likes of Hannibal, Garner, and Diallo into the fold.