If a manager is unfortunate (fortunate?) enough to be fielding separate teams at the senior, u23 and u21 level with full subs benches they would have 45 players. The current senior training field will only accommodate 42 players. Also, there are some teams trying to train up players but they are not yet good enough to replace a more senior player. With the current number of spaces some of these players will wind up missing out on training. I'm not sure whether the number of training spots increased with the introduction of the youth leagues or not.
If each coach has a limit of 5 players at the senior level, then it would make sense to increase the number of players eligible to be trained to 55.
Granted I doubt there are many teams who have the finances to support 55 non youth players so maybe a number between 45 and 50?