Nearly half a million Switch Pro Controller users have taking the controller over to Steam for some PC fun. This data comes from Valve themselves, who broke down what controllers users are enjoying games with.
The Switch Pro controller arrived in 2017 and players immediately began attaching them to their PCs. At the time, support was mostly limited to basic Steam Input remapping; meaning the UI did not match the physical device and features like motion control and rumble were not available. In May 2018, a Steam update enabled the full feature set of the device, added matching artwork in the UI, and improved the overall experience. The result was an acceleration in Switch Pro controller registrations, and a rise to the 7th most popular controller type on Steam.
As you can see, the chart also shows hundreds of thousands of players using controllers from the SNES, GameCube, N64, and more as well. There's a lot of Nintendo love going on with Steam fans!