I used to do this more with Civ 4- huge maps with one other civilization, or duel maps with every civ in the game.
I've tried messing with the number of civs and CS in Civ 5, and I find the default values tend to create the most balanced, fun games.
I recently tried one game with the max number of city states, and only a couple civs. Well, one of the civs was Greece, and before I knew it they were allied with pretty much every CS in the game. When Greece declared war on me, I was toast.