It's a massive undertaking. How many players do you think you need to sign? 8, maybe?
You've also got a load of loanees who don't appear to want to return to the club.
I'll give you a quick rundown.
Guendouzi and Mavrapanos are already gone.
Saliba is expected to be reintegrated into the first team, Ornstein has reported Nketiah is now expected to pen a new deal (this was like 30 mins ago), Torreira is also expected to leave, Lacazette is going, Pepe is going, Nelson has 1 year left on his contract, as does Maitland-Niles (also expected to leave), Bellerin has made public his desire to stay at Betis permanently so let's see, Runarsson and Mari don't have futures at the club either.
We've (unofficially, but pretty much done) signed Marquinhos from Sao Paulo who is a 19-year old winger, but I don't expect him to be with the first team.
Based on this, I expect us to sign 5-6 first-team players. Baring in mind a lot of those players I've mentioned weren't getting minutes/weren't that valuable to us, anyway.