Usher sold 11.867m in 2004 according to Mediatraffic. Norah Jones' "Come Away With Me" sold 12.2m in 2003, which Adele will blow out of the water by the end of November or end of December, not even including the holidays.
She will do more than 20 million in the end. Set Fire to the Rain still has to impact American radio and it will undoubtedly be another 150m plus A.I. smash. And with all the Grammys, BRITs, etc she's going to win, she will get there. I suspect it will be the highest selling album of this decade as well (worldwide). This is the most successful album for a solo female artist since Come On Over.