649 as of Gen 5, and it's confirmed that this Gen will have over 50, after this gen, we'll have over 700 Pokemon.
To be a little more clear, they only said we'd have "over 700" Pokemon. People have been overly literal and cynical and said that this only really confirms 52, but the smallest Pokemon gen to date had exactly 100 new Pokemon; fewer than this would greatly surprise me.
Now, I'm doubting more than 150 (because then they'd have advertised over 800), but 52 is the extreme lowest possible number - not the expected number.
All said, though, modern games have "regional Pokedexes", which give you a couple of hundred Pokemon indigenous to the local area - you'll only have that many to worry about prior to the postgame, not the whole 700+. In the postgame, you get the National Pokedex, which includes every Pokemon ever.
Creator of Jay's Journey (see quote!) "It's not ten years old! Therefore, it sucks!" - Nostalgia whores everywhere