https://www.gamespot.com/articles/steam-sets-new-concurrent-user-record-probably-tha/1100-6455965/

Steam, Valve's massively popular PC gaming platform, has reached a new record for concurrent players. Today, January 8, the service exceeded 18 million concurrent users for the first time ever. This is up from 16 million in October 2017 and 14 million in January 2017, so that's a lot of growth very quickly. Bear in mind that this figure is how many people were logged in at the same time; they might not have been playing a game.

Whatever the case, Steam's public data shows that the number of people logged in simultaneously today hit 18.4 million. The hugely popular Battle Royale game PUBG was far and away the most popular game on Steam today, with 2.9 million peak concurrent users. So that means around 16 percent of Steam's users today, at peak, were playing PUBG, which is pretty wild to think about.