What David Watts said - it only depends on one thing, what you like and enjoy in your baseball world. Nothing you do in OOTP has to mirror real life in any way, shape, or form if you don't want it to.
I have one league with 12 teams. Top 2 in each division (2 div) make the play-offs. Maybe it seems far fetched or not realistic, but I do it because it suits my style of play and makes the game more enjoyable for me.
I also ran a 16 team league (2 sub-leagues) with 8 teams each. Top 4 in each league made the play-offs.