While the number of games doesn't exactly mean much in isolation, a large amount of them have been making their way to Linux recently. Valve's first-party library is an obvious addition, as they have been jaded with Windows since 8.x scared just about anyone interested in back catalog support with their “Desktop as an App” attempts to isolate the Win32 APIs. Other developers have been following suit, especially since engines are being designed cross-platform as of late.
Milestones can be interesting, though. In this case, Steam crossed the 1,500 mark in games for Linux that are hosted on its service. Some equate this to “there exists 1500 games for Linux”, which isn't quite right, but the distribution platform is definitely a behemoth in the industry. It is the default way to purchase many new titles, and is a Linux host for ARK: Survival Evolved and Shadow of Mordor.
Unfortunately, I couldn't find anyone who listed what the 1500th title was. Sorry!