Bluwizard nailed it. Right around 12- 13 is the perfect number of players. You may want to adjust up or down a player or two based on what your're training (if your're training). You want your guys around 50-70 minutes to maintain peak Gameshape, though players can vary slightly.
In case it helps, here's an example of how i set my lineups been doing 1 position PG training.
Game 1:
Trainee 1- Trainee 1- Trainee 1
SG 1- Trainee 2- Trainee 2
SF 1- SF 2- SF 2
PF 1- PF 2- PF 2
C 1- C 2- C 2
Game 2 is the same except I swap Trainees 1 and 2
Game 3 (Cup/scrimmage)
Trainee 3- Trainee 3- Trainee 3
Bench Player A- Bench Player A- Bench Player A (usually my best bench guy since he'll get 48 minutes)
Bench Player B- SF 2- SF 2
Bench Player C- PF 2- PF 2
Bench Player D- C 2- C 2
The positions of the bench guys don't really matter for a meaningless scrim game.