Blizzard has announced that Legion sales on Day 1 were 3.3M copies. 

 With Azeroth on the brink of destruction, heroes from around the world were summoned to drive back the Burning Legion’s armies last week with the launch of World of Warcraft^®:  Legion^™, the sixth expansion to Blizzard Entertainment’s acclaimed massively multiplayer online role-playing game. Today, Blizzard announced that players have forged a truly formidable defensive front—as of the expansion’s first full day of launch on August 30, more than 3.3 million copies of Legion had sold through, matching the all-time record achieved by previous expansions and making it one of the fastest-selling PC games ever.^† In addition, World of Warcraft’s launch-week player concurrency climbed to its highest point since the 2010 launch of the Cataclysm ^® expansion, as champions from around the world united to strike a mighty first blow against the fel invaders. But the battle has only just begun.