https://steamspy.com/

Steam Spy is a website created by Sergey Galyonkin and launched in April 2015. The site uses an application programming interface (API) to the Steam software distribution service owned by Valve to estimate the number of sales of software titles offered on the service. Estimates are made based on the API polling user profiles from Steam to determine what software titles (primarily video games) they own and using statistics to estimate overall sales. Software developers have reported that Galyonkin's algorithms can provide sales numbers that are accurate to within 10%, though Galyonkin cautions against using his estimates in financial projections and other business-critical decisions. Due to changes in Steam's privacy features in April 2018, Galyonkin had anticipated he would need to shut down the service due to the inability to estimate accurate numbers from other sources, but later that month revealed a new algorithm using publicly available data, which, while having a larger number of outliers, he still believes has reasonable accuracy for use. - Wikipedia