How many pokemon are obtainable in XY
125 from older games are not obtainable in X/Y. Also you can't get Diancie. So you can get 593 Pokémon from the 719 in all.
You can take a look here which Pokémon are only available at a speciffic version: Pokemon X
Pokémon which are not available in X/Y (in German): not in Pokémon X/Y