The total as of gen 5 is 649. 29 new Ponemon have been revealed for X and Y, but the total number of new Pokemon for X and Y is still unknown. Based on past gens, I'd say at least one hundred. So yeah, a lot pretty much.
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Unknown at this point in time. You were right G/S/C had 251 in the National Pokedex.
Current National Pokedex count (generations 1-5): 649
A currently unknown number of new Pokemon will be added in X and Y. Seems likely we may get about 100. There seems to be about 20 of the new ones known about right now, bringing the currently known total Pokemon to about 670.
Be aware that not all will be catchable in-game. Most will have to be traded for, or transferred from past games (if they figure out the solution like many of us have).
I gave up and did the 3DS transfer anyway... 3DS XL FC: 2492-4470-1418 / White 2 FC: 0047-4149-5185 / White FC: 0348-0626-6780