Is there every Legendary Pokemon in Pokemon X and Y?

Answer 1

In Pokémon Y, you can find the Legendary Pokémon Yveltal at Team Flare's lab in Lumiose City (Xerneas if you have Pokémon X instead), Zygarde at the Terminus Cave, Mewtwo at the Unknown Dungeon near the Pokémon Village and one of the birds inside Sea Spirit's Den at Azure Bay after a few encounters in the wild.

You can get Moltres if you picked Froakie, Articuno if you had chosen Chespin or Zapdos if Fennekin is your starter. All of them are only available post-game except for Yveltal.

The Legendary Pokémon Hoopa, Diancie and Volcanion can only be obtained through an event.

For all the remaining Pokémon, the Pokémon Bank and Poké Transporter apps are required (unless you find someone willing to trade them with you).