Report: Team Fortress 2 has had over 50 million players
Steam leak gives us very accurate ballpark numbers of the biggest games on the platform.
The rise of PUBG might lead you to think it's the biggest game on Steam historically, but that's not accurate, even if the competition is slightly skewed (with some games being free-to-play or very cheap). The biggest game, according to this post on Ars Technica, is Team Fortress 2. The free-to-play title first launched over ten years ago, and according to these numbers has amassed over 50 million players over the years. Second in the list is another Valve staple in Counter-Strike: Global Offensive with over 46 million players. PlayerUnknown's Battlegrounds is the most recent release in the top with more than 36.5 million players in third. Another 2017 release, free-to-play survival game Unturned ranks fourth with over 27 million players, and Left 4 Dead 2 rounds out the top 5 with more than 23 million players.
These numbers were leaked as a result of a slight hole in the API that has since been patched up. But hopefully, Valve will provide accurate numbers in the future as they do with concurrent player counts.