They can't possibly fake that many pokemon, seriously.
The only ones that may be fake are the starter's because the issue is confirmed to feature only 18 pokemon.
Though I may be wrong.
I think Dento's the main gym leader. In the trailer he is originally the only one there, and the other two appear when you get the badge. So I'm assuming you fight them all.
You can see two of them in one of the trailers, I believe.
Dento was the one who gave the badge to the player, so I thought that there would be a triple battle. But then again Dento could be the one you battle last.