As far as I know, Larian does not often give exact numbers on their sales, getting an exact amount on DOS2 is impossible, it is all speculation at the end of the day unless I'm missing something. Even the 1million mark for BG3 was not a number mentioned by Larian.
Here are some rankings showing where dos2 was at.
https://store.steampowered.com/sale/2017_best_sellers/https://store.steampowered.com/sale/winter2018bestof/https://store.steampowered.com/sale/2019_top_sellers/
