The dust jacket around Ace of the Diamond’s 41st volume* reports that it presently has 20 million copies in print:
Pretty cool when you consider that the series’ best-performing volume on the Oricon charts to date, volume 40, capped out on the charts at 276,087 copies sold. Even if we assume that all 41 volumes of the series capped out at a similar number (instead of taking their lower chart figures), that gives the series a total sales figure of 41*276,087=11,319,567 copies, about 8.5 million copies short of the publisher total. So yeah, long tails and/or other ways Oricon can underreport figures are definitely in play to a large degree, as per usual.
*Aka: the Sanada Shunpei Super Show.