Three is my self-imposed capacity.
The preseason activities, the draft, and the end-of-season activity and planning for the next season are the most time consuming parts of the game for me. In season game management and minor league management is just a couple of minutes every game cycle.
My fear is that a fourth team would increase the risk of the more time-consuming parts of the game for me clashing across multiple worlds, and Murphy's Law dictates that it would happen at a time least convenient in real-life for me to handle. I've pretty much avoided that with three teams.
Cost is secondary. I could afford a fourth team if I wanted provided I open the credit card bill before my wife does and not have to deal with the "what's this charge" questions.
Don't think I have any suggestions for fixes or improvements to the game that would give me a reason to pick up an additional permanent team.