I'm not sure how many were not revealed. But Archia and Maxie having Mega Hondoom and Mega Gyarados respectively would be stupid imo, so i 'm pretty sure we are Getting Mega Camerupt and Sharpedo.
And I'm pretty sure they don't want to reveal all the Megas, Its very likely we are getting Mega Milotic and Flygon.
Also, I think Mega Audino is just a new form, not a Mega.
Do you know how many megas were not revealed in XY but eventually made it into the games? I know gardevoir and gyarados, but I can't remember there were how many more.