https://www.pcgamer.com/steam-now-has-90-million-monthly-users/

In a 2018 year in review post, in which Valve outlined plans for Steam in 2019, the company also provided some rare insight into how many people use Steam every day and every month. And the numbers are, unsurprisingly, big: in 2018, Steam averaged 47 million daily active users and 90 million monthly active users. It also broke the record for concurrent players early in 2018, with 18.5 million.

While Steam is clearly growing fast—it recently passed 30,000 games, and just look at the amount of data Valve has served up—it's interesting that that concurrent users are actually down since it broke that record in early 2018. This chart from SteamDB shows daily users declined through summer 2018, before rebounding to around 17.5 million at the beginning of 2019.