Launched in 2015 by Sergey Galyonkin, director of publishing strategy at Epic Games, Steam Spy makes use of an application programming interface (API) to scan through Steam profiles and collect data on who owns what game, in order to figure out accurate sales estimates. According to software developers, these estimates are typically accurate to within 10%, though the man himself has advised against using these figures for anything super serious, like financial projections.
Later that month, Galyonkin mentioned that he’d been experimenting with alternative techniques. Just today, it seems that whatever he had attempted is now functioning, albeit in a slightly different way. Rather than owners, these figures are labelled “Players (Experimental)”, which suggests they may not be as accurate as before. Galyonkin takes pride in his work, however, so it’s likely in at least a serviceable enough state for the estimates to be reasonable, otherwise, they probably wouldn’t be available.
Some examples of the number of “players” for games on Steam include Dark Souls 3 at 3,295,000, and Hollow Knight at 959,000. You can visit Steam Spy if you’d like to browse for yourself.