How long before release did XY stop revealing Megas? Actually I only recall they showed off six (Blaziken,Absol, Mawile, Mewtwo Y, Ampharos, and Lucario) in corocoro, the Kanto Megas minus X were shown in Pokémon, and later Charizard X in Origins. GF is spreading the Megas of these games much more, but also revealing more it seems. Thus far we got Mega Sceptile, Swampert, Audino, Diancie, Sableye, Altaria, Lopunny, Metagross, and Salamence officially revealed (as the Latis were discovered by looking into the data, and Slowbro was an accident). That there is ten. So I think we have enough shown already. So they can keep the rest secret.
Btw on speculation of how many Megas will be early in the game. Since it seems Megas took the place of new Pokémon for Kalos to make up for the low number of real new Pokémon, perhaps Hoenn will have them early to make up for a possibly smaller than XY's dex.