Cristiano Ronaldo is the latest addition following his sensational return from Juventus and, as has been well documented, he is wearing the no.7 shirt with Edinson Cavani switching to the no.21 vacated by Daniel James, in the wake of the Wales international's switch to Leeds United.
The rules state that each club may register 25 first-team players aged over 21, with a minimum of eight of these classified as 'homegrown'. Any number of under-21 players can also supplement the squad and, of course, these include regular first-teamers such as Jadon Sancho and Mason Greenwood.
Please note this is not necessarily restricted as the only players available to Ole Gunnar Solskjaer because, as already mentioned, other youngsters can also be used to supplement the squad.