Pokémon X and Pokémon Y

Honestly, at this point, it's not such a bad idea to mix and match regions. Hell, playing SoulSilver recently, I forgot half of the Pokemon in Johto existed since I haven't played Silver in probably 8 to 9 years. It's not such a bad idea to revisit some of them.

Didn't every game except Black/White mix regions in the regional pokedex?

They can still do it, but 52 would be a tiny number of new pokemon. Red/Blue added 151 (obviously). Gold/Silver had 100 new pokemon. Ruby/Sapphire had 135 new pokemon, Diamond/Pearl added 107 new mons, and Black/White added 156 new pokemon.

So really, anything less than 100 new pokemon would be a new low. I don't expect it to be breaking any records either, since the entire battle system is being remade into 3d, whcih means they need to model all the new moves and stuff in addition to the new pokemon models, making for extra work. (The 649 current gen pokemon are already done, the X/Y designs are reskins of the models from pokedex 3d pro.)