There is another version of this map that I did with fixed starts for 12+1 nations (so that's 12 Land starts and 1 Water start). So if you are going for 12 nations, then this second version will be much better than the 13+1 version which you are currently using.
But of course you will have problems if anyone picks a water nation, as if only 11 land nations start then some nations are going to have a lot of room to expand into where the missing nation would have been (and will get a big advantage compared to nations in other areas. As pointed out above by Peacekeeper)
The link for the 12+1 version is here
http://forum.shrapnelgames.com/showpost.php?p=775421
and on the llamaserver it's just below the map currently selected for this game (it's called "Riverlands for 12+1 (Andalusian Sun - The Restart")