XY had 28 in total (counting both variants of charizard & mewtwo) (+ the lati but event so shhhh)
We'll put aside Mewtwo's initial unveil since it was literally nothing but ??????????? for like 4 months straight
August: 6 (Mewtwo Y, Blaziken, Ampharos, Mawile, Lucario, Absol)
September: 6 (Mewtwo X, Garchomp, Charizard Y, Venusaur, Blastoise, Kangskhan [not in corocoro])
October the games came out, and we had already had leaks going for a solid week, but if we hadn't coroco leaked 4 more (Tyranitar, Gengar, Aggron, and a formal showing of Kanga).
Though if how they handled the pokemon unveils (I'm pretty sure we knew the vast majority of them), had they shown Megas off earlier we probably would have seen more.