Bluehole hasn't officially announced the numbers yet, which were gathered using existing owner data on Steam's API. It's not too hard to believe considering just last month Bluehole announced selling 4 million copies of the game.
This is pretty phenomenal given the fact that Battlegrounds also recently beat Grand Theft Auto 5's concurrent players count. Battlegrounds won't even properly release until Q4 2017, which could mean we won't see a full version of the game until December 31 at the latest.
Still, with monthly patches, and a pretty regular update schedule that will bring in highly-requested features like first-person servers, Battlegrounds is proving to have some serious legs.