So, One Piece has an exposition in Tokyo Tower will open in March 13th. The thing is that they gave a brochure where they show One Piece print numbers since 1997 to 2014(In reality it seems that it only goes as far as 2013).
Here the pages of the brochure with that info:
Here is the above data translated, formatted and with some additional info:
! 1997, 1st year - 300k copies printed, 1 volume, 300k copies per volume! Note that 300k is the first print of volume 1, because it was released in 24 of December, a second print would be accounted in the next year.
1998, 2nd year - 6 Million copies printed, 6 volumes, 1 Million copies per volume.
1999, 3rd year - 17 Million copies printed, 11 volumes, 1.55 Million copies per volume.
2000, 4th year - 37 Million copies printed, 16 volumes, 2.31 Million copies per volume. An anime boost of 767k copies per volume printed, and this only if we don't account the 2 first months of anime boost in the previous year.
2001, 5th year - 54 Million copies printed, 21 volumes, 2.57 Million copies per volume.
2002, 6th year - 73 Million copies printed, 26 volumes, 2.81 Million copies per volume.
2003, 7th year - 90 Million copies printed, 31 volumes, 2.90 Million copies per volume.
2004, 8th year - 99 Million copies printed, 35 volumes, 2.83 Million copies per volume. First decrease in the average of volumes per volume, this represents the year of the end of Skypiea, Davy Back Fight and the beginning of Water 7.
2005, 9th year - 112 Million copies printed, 40 volumes, 2.80 Million copies per volume. Reached 100 Million copies printed in February of 2005 with volume 36. With this One Piece is the fastest title to reach 100 Million copies in print. Dragon Ball was only able to reach 100 Million after its ending and it run for 11 years and with 42 volumes.
2006, 10th year - 125 Million copies printed, 44 volumes, 2.84 Million copies per volume.
2007, 11th year - 139 Million copies printed, 48 volumes, 2.90 Million copies per volume.
2008, 12th year - 153 Million copies printed, 52 volumes, 2.94 Million copies per volume.
2009, 13th year - 177 Million copies printed, 56 volumes, 3.16 Million copies per volume.
2010, 14th year - 216 Million copies printed, 60 volumes, 3.60 Million copies per volume. Reached 200 Million copies printed in November of 2010 with volume 60.
2011, 15th year - 257 Million copies printed, 64 volumes, 4.02 Million copies per volume.
2012, 16th year - 281 Million copies printed, 68 volumes, 4.13 Million copies per volume.
2013, 17th year - 302 Million copies printed, 72 volumes, 4.20 Million copies per volume. Reached 300 Million copies printed in November of 2013 with volume 72. First on only title reaching 300 Million copies printed!
2014, 18th year - 320 Million copies printed, 76 volumes, 4.21 Million copies per volume.
Here the rest of the Exposition brochure: