According to SteamSpy, PUBG has reached the 30 million copies sold mark, which is quite an impressive feat. Although, I still don't understand the appeal of the game at all.
As a reminder, it reached 10 million September 5.
Next milestone was 20 million November 7th.
As of late December, they boasted 30 million players total -- including XB1.
To compare the pace, it took the game:
5 months and 14 days to reach 10 million
2 months and 3 days for 20 million
3 months and 6 days for 30 million
Last month it was at 27 million. Probably past its peak, but still chugging along at an insane pace.