This can be solved with a simple Google/Wiki search, but for the sake of simplicity:
All usual ATRL sources indicated it selling over five million in 2011 alone usually between 5.4 million (sources like
Mediatraffic aka two German gay men and their dog) and 5.8 million (
MusiCharts). Reputable sources such as a 2012
Live Nation press release showed the album had sold approximately six million copies about one year after release; the highest conventionally cited source is a
Billboard article claiming sales had surpassed 8 million in October of 2011, but it's likely they meant shipments. It is reasonable to conclude, based on this information and residual sales over the years since, that it has sold about 7 million copies, and I'm sure there are sources and estimates out there that support such a figure.