There aren't many stats on actual Linux usage for cross platform games. The only public data is from Humble Bundles, and it was for indicated usage (i.e. you could select what platform you intend to use the game on while buying).

Even if it's 10% - it's a lot of users.

There was some article, that availability grows gradually. I.e. if too many games will come out for Linux at once, the market will be too flooded with them, which will in turn hurt both developers, and users.