I prefer flat out 4max per sector, race nonwithstanding.
If the AI bugger out-starbased you, you'll just have to take it or destroy his starbase to make room
It would also help the AI, I think, if the 4 places for starbases in the sector would be set in advance when generating the map. also, no big hurt to gameplay there. I always hated seeing starbases all cramped up together