Indeed some great answers here. Mystery Leagues are so much better but you have to pay attention to EVERYTHING! I love the Mystery Leagues and I will only occasionally play in non Mystery Leagues.
I used to run around 4 at a time, but I found that I would end up losing interest later in the seasons of the struggling teams if I had other "better" ones. Due to finances and more time constraints, 3 is a good number if I can afford it, spaced out by at least a couple of weeks each.