I'd like to say that most gods are unrestricted as long as there is a feas. Asible reason for how your character came into being. Also they HAVE to be a demigod, half god, half human.
In terms of how many characters. As many as you want, but I'd stick to at most 3 characters. As long as you can manage all of them.
Hey if you are still looking for people I would absolutely LOVE to join this RP. I am a huge fan of Greek mythology and the Percy Jackson series. If you could save a character spot for me as a Son of Hades that would be awesome!
Poseidon would definitely take Athena's side because in Greek Myths Athena, Poseidon, and Hera all teamed up to capture Zeus and make him give up the throne (which is what this RP is based on). Basically their hate for each other is unmatched to their hate for Zeus on the throne.