Humans are generally necessary because it gives the audience someone they can more readily relate to.
If it's a story then I don't think there are any limits to the amount of humanoid versus non-humanoid species, since it all comes down to a text description. It's more limiting in film obviously, even with CGI (not to mean comrade_general's ingenuity) the represented characters most often have a roughly humanoid shape, particularly in the face.
When asked why so many of the aliens in the various Star Trek series looked humanoid the producers responded, "You find us non-humanoid actors and we'll cast them."