From my experience, when training 2 positions, you can safely train 5 players per week... 6 players is too hard, and with 5, they'll almost never lose training minutes...
For example, I had games where I put my center to play entire game, and I put him on starter, back-up and reserve position for the game... But my team was winning by 40+ difference, and my coach put my SG to play entire last quarter at the center position...