Why would the Ascians and Beastmen (as a whole) have specifically thirteen primals each? Or am I missing something?

Just because the Eorzean civilization has thirteen (if you're counting Hydaelyn), why would it have to be the same way for the others? It'd just seem they'd have however many primals as they have "gods." Which could be one for each beastmen tribe, could be multiple for some tribes that worship multiple gods/primals... Besides, the beastmen are different races from each other, why would all of them be lumped in together?

For that matter, do we know there are only thirteen gods/primals for the Eorzeans civilization? I mean, there could be some cults, some outcast gods that it's blasphemy to talk about... etc. etc...