One Piece volume 65 will be released in Japan on February 3rd; 4 million volumes will go on sale beating the previous record of 3.8 million volumes, also held by another One Piece volume. If you do the math that means that around 3% of the Japanese population is reading One Piece at the moment, most likely even more.
One Piece volume 65 cover. The volume will be released at the end of this week
Selling more than a million volumes is considered a huge success for a manga in Japan. There are not many manga series that can achieve such a feat: Nodame Cantabile, Bleach, Naruto, Full Metal Alchemist and Kimi no Todoke.
Regarding total volume sales since the beginning of a manga series, One Piece is the number 1 manga series in the historical ranking having sold 250 million volumes in Japan, followed by Kochi no kame and Dragon Ball both having sold nearly 200 volumes.