There were few hints and much more detailed ones but from in-game:
(Sorry if NA lore name is different)
Dalamud was launched by ancient arag empire and is man made
Each tribe has individual gods (beastmen having primals, human race having the twelve)
Van Darnus carries blood from ancient arag empire.
Theologian from Ishgard has been abducted by the empire.
I wont allow any more gods to be called upon (Garuda's script)
Power of the crystal is sent to Dalamud
Dragon race also falls under what the Eorzean calls as beastmen tribes, so.....
Niddhogg isn't a god but is a dragon in highest place of the dragon race, also Midgarsom is special but isn't a god.
For humans there are many kings depending on the era right? That's about it on what I can say for now.
Now we know that midgardsormr is not a primal himself but a very powerful beast in the game. Also by yoshi-p saying that each tribe has individual gods like beastmen having primals and human race having twelve. Then i think it save to say that just other world for saying "God" like us humans call them twelve because there 12 of them and the beastmen call them primals because that what word they use so at the end the twelve are primals and could be as strong as them as long as the world have enough power to summon there full power.
Seeing how in 2.0 free company can summon primals it save to say that us human can make also summon our own gods. Also it seem in 2.0 they be a way to summon the primals without using the power of the land by other means seeing how many free companys would be summing a lot of primals and yoshi-p said before you allow to use it you have to fill a bar so i'm guessing that bar would be power to summon them.
Sadly yoshi-p did not give anymore information because he does not want us to find 2.0 boring we got a lot of information tho.
Also Niddhogg isn't a god but is a dragon in highest place of the dragon race