The fact that Valve's Steam Deck portable console already supports more than 12,000 games from the Steam library was pointed out by the GamingOnLinux website.
As you know, Steam Deck uses the Linux-based SteamOS operating system. At the same time, the total number of Linux-supported games on Steam is 13,058 titles. So, it turns out that the lion's share of them work fine on Steam Deck.
Interestingly, there are not many more games for macOS on Steam - 18,749 titles. But there are 89,317 Windows games.