https://www.gamespot.com/articles/pubg-passes-15m-concurrent-users-dev-considering-s/1100-6453560/

Playerunknown's Battlegrounds, one of the biggest and most popular games on planet Earth right now, continues to grow. The game has now crossed 1.5 million concurrent users on Steam. That's a few hundred thousand more than 1.29 million that DOTA 2--a free-to-play game--had at its own highest peak in March 2016. DOTA 2 and PUBG are apparently the only two games in Steam's history to pass 1 million peak concurrent players.

You can see more peak concurrent user figures for Steam games below, via analyst Daniel Ahmad. PUBG's sustained performance is pretty remarkable.