Obviously you want a full 25 man squad, goes without saying. How often have we used all 25 though?
17 is too low.
21 or 22 is required.
to answer the question, you really hope you don't need to use them all but they have to be there
3 keepers, you hope you don't need 2
8 defenders if you play back 4, you really hope 1 oe 2 hardly play
etc
etc
HOWEVER
it is likely uefa will have to change this rule before long due to EU courts and we could see an end to association trained players
Mount might be worthless to anyone but Chelsea in future and anyone signing him has in effect another none homegrown player.
if we had to have 8 club trained players in the squad we would be royally screwed over.
the market for 18 year olds would explode.

