I've been looking at Xbox One 2015 data; from October 10, 2015 to the end of 2015, it sold a total of 4,614,502 consoles. If Series sells the exact same amount in the same period of 2021, it will have sold a total of 9,35 million consoles in 2021. Since Series 2021 is ahead of Xbox One 2015 (4,61M vs 3,65M at this point in the year) and there is no reason to think it might lower its advantage much, I think Series should reach 10 million units sold this year.