Mantan Web announced that the successful manga Attack on Titan (Shingeki no Kyojin) will go on sale with 2.2 million copies for its 12th volume, a number that has already surpassed the barrier of 2 million copies in circulation.
To give you a better idea of the speed with which the manga gained popularity, this number is 55 times higher than the first volume that was released in March 2010 with just 40 thousand copies. The number of sales of manga volumes in Japan has already exceeded 28 million.
Finally, a curiosity, the other Japanese manga that had a first printing of a volume exceeding 2 million was One Piece by Eiichiro Oda. The 71st volume of One Piece in August 2013 had 4 million copies in the first printing.