https://www.pcgamer.com/steam-had-135-million-new-first-time-purchasers-between-january-and-may/

Valve's business development head Jan-Peter Ewert says Steam had 13.5 million "new first-time purchasers" in the first few months of 2018. 

Speaking at the Business Conference for Games Industry event in Russia this week, Ewert tied this figure—around 3.375 million new buyers per month between January 1 and April 30—to Steam's recent increase in supported currencies. Ewert suggested Steam's increase in supported payment methods is designed to benefit developers, and that this figure has risen from 40 in 2014 to over 80 methods this year.