Hello! Unfortunately I couldn’t find a list of twelve, and I also remembered twelve, but in Malory there are eleven kings (or leaders) that are hostile and rebel against Arthur when he is crowned:
– King Brandegoris;
– the Duke of Cambenet;
– King Clariance of Northumberland;
– the King of the Hundred Knights;
– King Lot;
– King Uriens;
– King Idres of Cornwall;
– King Cradelmas;
– King Agwisance of Ireland;
– King Nentres;
– King Carados.
Basically Arthur’s sisters’ husbands (Lot, Uriens and Nentres) and less known people!
Arthurian Legends 