Not many indeed. There's a switch in your settings to only show you games for your designated OS, or there are symbols below the games' vignettes that tells you which OS is supported: win / mac / steam

Frustrating is famous games from the 32 bits era, where they would be available to Mac but work only on macos pre-10.14 and not from 10.15 when macos went fully 64bits. Which means on top of reduced availability, some of this availability would only work on a mac from 2010 or so.