Best answer: According to ProtonDB, a website that tracks game support with Valve's Proton compatibility layer, almost 16,000 Windows games are playable.
What is Proton?
Windows-based software and games aren't natively compatible with Linux (and other operating systems) unless the developer includes the required files. This is where packages like WINE (WINE Is Not an Emulator) convert code so that Linux is able to run apps and games without requiring some form of emulation.
Valve is taking this a step further with Proton. Utilizing WINE and other community-built tools, the company has been able to package together tech that can automatically allow Windows games to run on Linux through Steam. It's called Steam Play and lets you essentially click "install" on your favorite non-native Linux game and enjoy it as you would on Windows.
Performance in games used to be a major blow on Linux, compared to Windows, largely due to compatibility and official support. Valve changed this with Steam Play. Proton and other open-source tools work together to make the whole process of downloading, installing, and playing Windows games straightforward.
And the Steam client handles everything for you, including the installation of Proton itself.
Check game support with ProtonDB
ProtonDB is a website dedicated to tracking the performance of games on Linux using Valve's Proton (and Steam Play) tech. Games are rated: borked, bronze, silver, gold, platinum, and native. Native is given to games that do not require Proton or other tools for them to run on Linux.
Borked are games that don't work at all, platinum work almost perfectly through Proton, and silver and gold are awarded to titles that may have features that do not work, but otherwise are playable. Visiting a game page will show reports from users, as well as required steps to get games (or features like online play) working.
Thanks to Proton and the incredible Linux community, more than 15,000 games technically run on Linux. The OS is now a viable option for PC gaming, and with Steam Deck, things are only set to get better for the open-source community. Using the best graphics card won't be limited to just Windows for gaming.