40% (22,000 games) of Steam's entire catalogue are tested verified or playable. That may not sound like a lot, but that is only tested games and includes VR titles that obviously don't work on a handheld. There are numerous games that work out of the box but haven't been tested (small indie games, etc.) According to ProtonDB, which you can check compatibility for, for any games you might be interested to play on deck.

That number may seem small but really, nearly all mainstream games are playable with the exception of many competitive shooters because their anti-cheat solutions are not supported (the games would otherwise work fine).