I usually have around 25 at the start of the season, and by jan-feb this is usually down to ~15 due to injuries, sellings and demotions to the reserves.
Putting all the backups in the reserves might be a good idea if you have a small squad, but when you have a squad of ~60 players and only 15 of those in the u18 and 11 in the first team the reserves are going to be crammed with people.
Shure you can loan them out but you probably can't loan out ~20 players a season and this will lead to you having a few young guns not playing at all thus no improving.
Also when first team and reserves matches clash all the reserves are no shows for your first team, meaning you'll have to draw upon the "vast" knowledge of your U18 players to get you through the game.
Just my £0.02 worth