More than 2100 games from the Steam library now work on Linux and the Deck.
2100 Games On The Steam Deck, with Metro 2033 Redux and Resonance of Fate as Verified - Boiling Steam
https://boilingsteam.com/2100-games-on-the-steam-deck-metro-2033/
A new milestone for the Steam Deck, with 2000 games validated (playable and verified) now available on the first of April 2022
The real number is probably much higher. I routinely play games on my Arch laptop that are neither verified nor marked as playable.
Some of them are weird, I love Out of Space, play it with my wife almost daily, not only it has a native version but it works flawlessly on the Deck and even before that it worked flawlessly on my laptop for years, yet it's marked as Unsupported. I haven't checked other games because I usually rely on Protondb badges, but in any case the verified is not a good marker for amount of games that work on Linux, I played metro redux years before proton and it's only getting the verified now.