Four, it looks like. But of the games in the top ten that work on Linux, only one (GTA V) is not from Valve. Definitely seems like the year of Linux gaming on the desktop has not yet arrived, although we're closer than we used to be. Great work by Valve to get us this far.

Also worth noting that a number of actual top ten games are not on Steam. Valorant, league of legends, warzone, escape from tarkov, and Fortnite all are not available on that platform. (Some of these may not be top ten -- it's hard to know because we don't have solid numbers, but they are all in the top 20 of twitch viewership.)