Take into account that if you currently have any 18 years old players, next season he will disappear. In total, you can have 10 player in the Academy. 7 new players more 3 already "in" players. If you currently have more than 3 players under 18y/o, i suppose that you will keep them + a number of new players (less than 7) respecting the limit of 10 players into Academy.