In terms of the display area available, is there a practical upper limit on the number of Lists that fit well on a Trello Board?
Trello was designed to work on any size screen, so there is no technical or display limit to the number of lists you can have. The number of lists is going to depend greatly on your process, your device, and your window size, so there is no correct answer. Things are more difficult to mentally parse with more lists and cards (everything is meant to be seen "in a glance"). You might try to break your process up into multiple boards if that's becoming a problem.