Limit alliances to no greater than 1/3rd total regions
People will always find ways around hard limitations such as that. You might as well say, make sure every realm can only have 1 or 2 alliances. Any limitations of that sort would just lead to creative work arounds. For example, having an informal block of 4 realms (e.g.) with realms 1 and 2 as allies, and then realms 3 and 4 as allies, with the armies of all 4 rotating in and out of the theatre of war or attacking from different sides.
Artificial limitations to diplomacy don't really work very well.
