SteamVR is the dominant VR platform on PC, used to play VR games on Steam. As it's not common for VR headset makers to disclose sales numbers, the monthly SteamVR hardware is as close as it can get to any real numbers.
As the focus shifted to standalone devices, the standalone market is only partially represented. As some standalone devices can be used as a PC VR headset, they are included in the SteamVR data. But the portion of it's users that doesn't use SteamVR is not represented in the data and we can only guess on how big it is.
SteamVR Headset Adoption
This corresponds with the percentage of Steam users that have used VR headset in the month of the survey. The calculation mechanism has change in 2020 (which explains the spike)
We also edited this data to exlude Chinese users (by launguage setting). As Chinese users accounts for 20-50% of Steam users according to survey and the number vary greatly, since almost no headsets are on sale in China, we've adjusted the number to exlude Chinese users. This helped to smooth the curve a lot.
SteamVR Headset market share
Minor headsets and makers are omitted. The percentage that's missing to 100% is where all other headsets are.