That information is to take with caution, Terran Gregory, the In-game Cinematic Project Director for World of Warcraft said (after a little of torture) that for the moment 6 in-game cinematic would prepare for the release of Warlords of Draenor.
Remember than for the launch of Mists of Pandaria, 3 of them were there : The Battle of the Serpent’s Shrine, and the Horde & Alliance ones.
P.S : Of course, that number could change before the final release of Warlords of Draenor.
@Abrosiabi@Lugia_Blizz@ChristieGolden@DaveKosak I’d guess up to 6 story cinematics, though that may change as Alex said (still in design)
— Terran Gregory (@TerranGregory) 20 Mars 2014
The Battle of the Serpent’s Shrine