Just to put this to bed (unless I screw it up) you can have a maximum of 25 in the squad plus the (pretty much unlimited) youth players, you don't have to have 25 of course.
Of those 25 a maximum of 4 can be counted as Club Trained, if you have 5 for instance then 1 would have to be counted as either one of the 4 Association Trained players or if you also have 4 of them (taking up the first 8 slots) then they would have to go on the open list of 17. So you could have 25 Club Trained players but only 4 count as such and the other 21 would fill the rest of the slots.
Just to be awkward if you don't have a full 25 then the numbers change slightly in terms of homegrown.