Another thing to consider: Even with only one AI empire at the start, more will appear over the course of the game due to primitives reaching FTL technology and becoming empires in their own right. This however usually takes quite long and it is highly unlikely that those huge empires in your game are former primitives. There's probably another reason for why it looks the way it looks, but I just wanted to note that you will not have a static number of empires/"players" as you would in non-grand-strategy games.