Agree with this, people don't account for players going through slumps or others growing to fulfill their potential. We can say we need 10 players but then find out that players like Hoijlund have turned a page, Gore has grown into a proper first team player etc. We need to be rid of absolute wasters like Martial and then we have those we know that they will never amount to much like McTominay but there is no way we are signing 3 CMs in one window.
I think what we need will depend more on the departures. Martial looks done so we need a striker to take the pressure off Hoijlund and carry the weight for now, we then need a top CB to replace Varane, Maguire and Lindelof and a young DM to provide cover. The rest of the team's needs should be supplemented by the youth players - Mainoo could be the starting DM or partner to Casemiro if he stays, the fullbacks can do for one more year whilst Garnacho, Rashford and Amad can work the wide areas and maybe a new signing to replace Antony if he doesn't improve.