Mantan Web reports today that the 12th volume of Hajime Isayama's Attack on Titan manga, which is officially scheduled to be released in Japan on December 9, has 2.2 million copies for its first print run, breaking the 2 million record for the first time. The number is 55 times more than the first volume's first printing in March 2010, only 40,000. The 11th volume published in August 2013 got 1.85 million copies initially. The total sales of the manga series in Japan has reached 28 million copies. Obviously the number is still growing strongly.
The only other Japanese manga title which had over 2 million first copies is, as you guess, Eiichiro Oda's One Piece.The 71st volume of the manga published in August 2013 had 4 million copies in its first printing. So it is still a long way to go for Attack on Titan to be the top selling manga?