You might need a bit more than 12 because there are 3 games per week, and too many minutes are bad for form (called "practice" in BB), just as too few minutes are. I started with 18, sold one, and just picked up 3 more in the draft, and so I have started to trim back again.
You might experiment a little bit, since you might have players who have different skills, but who might be the best player in different situations.