I don’t know the methodology, but this article from about a year ago estimated 40 million Ubuntu users. https://www.bleepingcomputer.com/news/security/almost-40-percent-of-ubuntu-users-vulnerable-to-new-privilege-elevation-flaws/
Which compares to what, 2 million Steam Deck sales? 3 million? And how many of them remain active users? Doubt it’s even double digit percent.