It's very likely that the number will soon increase
Valve is looking very seriously at Linux as a genuine replacement for Windows and the company has put a lot of effort into it. An interesting way of seeing just how much they care is to check the top ten most played games on Steam.Many Steam users still see Linux as just another platform for the client, but it’s actually much more than that. Valve is betting its future on Linux and SteamOS, and anyone who is doubting that the company doesn’t know what it's doing should take a look at the most played games on the Steam platform.
Valve provides accurate statistics about what people are playing at any given moment and, right now, seven out of the most played ten games also have Linux versions.
They are as follow: Rust (FPS MMO with Linux support), Warframe (no Linux support), Counter-Strike (multiplayer FPS with Linux support), The Elder Scrolls V: Skyrim (no Linux support), Football Manager 2014 (football sim with Linux support), Garry's Mod (physics sandbox with Linux support), Sid Meier's Civilization V (TBS with Linux support), Team Fortress 2 (multiplayer FPS with Linux support), Counter-Strike: Global Offensive (no Linux support), Dota 2 (MOBA with Linux support).
Counter-Strike: Global Offensive will soon get a Linux version too, so the number will be eight shortly. Also, keep in mind that the list might vary depending on the users going online from different time zones.