https://www.gamespot.com/articles/playerunknowns-battlegrounds-tops-1m-concurrent-pl/1100-6453218/

PlayerUnknown's Battlegrounds is on a roll; after just hitting 10 million PC sales, it has now surpassed 1 million concurrent players on Steam for the first time.

At the time of this writing, PUBG hit a record peak of 1,028,275 players. It far surpassed the number of concurrent players playing Dota 2 (a regular occurrence now), but it's not high enough to snatch the all-time record from the game. Dota 2's all-team peak number of players is 1,295,114 (the highest ever recorded, according to SteamDB), so PUBG still needs to get 200K+ more concurrent players to truly unseat Dota 2 as the reigning multiplayer champion. Its creator, Brendan Greene, is confident that's a likely possibility; he suggested PUBG could hit League of Legend-level success in the future.