I wonder about this too. I am not quite sure but I think just one building is enough. but the trigger is something like %5 every turn, so it may net trigger for a long time. One time it took me more than 50 turns until quest desicion of the clinic building triggered after I finished it in my capital. One more example, I am playing as Barre who receives Old Earth Relic building from the beginning in every city. It has been more than 100 turns and the OER quest is still hanging out there waiting to be triggered :S I wonder if every other copy of a building in each city improves chances of triggering the quests?