The maximum stack outside a city is 12. I don't know if there is a limit inside a city, but I cornered one of the computer opponents in his last city earlier today, he has 35 units. This results in some bizarre situations, with catapults stacked for instance. Also, given the current state of the game, the AI doesnt appear to actaully have too pay for any of those units.
1. What is the stacking limit in a city?