With 3 games per week, the maximum is 6 players (if you are training PF/C). But that requires perfect distribution. You have two league games per week. If your best PF and C are only playing 48 minutes it could end up losing more league games. If a player ends up short of 48 minutes, he doesn't get full training. It really isn't known how much the penalty is, but it likely to be be greater than proportional. So a player who plays 40 minutes will likely get less than 40/48 of the full amount of training. You probably don't want your best prospects to end up short.
It is pretty easy to train 4 players, with backups that won't embarass you in league games. But you might decide it is too high of a salary for a 3rd string C who only plays in scrimmages. So if you are training PF/C you might use younger SF or SG as the 5th and 6th trainee, letting them get any leftover minutes. You could switch based on the skill. Everybody can use rebounding, IS and ID will be more beneficial to forwards.