There's a big difference between the amount of gens and the amount of Pokémon.
Do we know how many pokemon there will be?
They wouldn't. It was a hint at a flaw in the wording of your title.
That said, I'd say compare it to any gen but pre-BW2 Unova. They have pokemon from various gens, depending on how well they fit in. The fact that there are three dexes can hint at how there are way more pokemon obtainable than usual, but it could also just be a categorizing gimmick of the pokedex. Sinnoh had no such feature, but it was split into west and east Sinnoh for Shellos, which could have been enough reason to justify east and west dexes.
http://jojodee.deviantart.com/ -- Fakemons and other stuff
Having an Eevee pretty much guarantees appearances by generation 1, 2, and 4 pokemon. With the free torchic, guaranteeing gen 3 will be represented, but I don't recall seeing anything with gen 5 pokemon.
So ready for: Phoenix Wright 5, Pokemon X, Senran Kagura Burst, Tales of Symphonia Chronicles, Tales of Xillia 2, and Professor Layton VS Phoenix Wright.