you seem to be unfamiliar with how CrOS works. users don't pick versions, they pick channels, and channels automatically update to the latest version. so there is no answer to your question because it doesn't make sense. it's like asking what is the adoption rate of the current date and time: everyone is on the same version because they have no control over it.
hence my recommendation that you review the release schedule to see what the current version is, and target that.
if you want more information for how Chrome channels work, please review the documentation: