Unlike the mobile Android operating system, Android TV isn't a subject Google often gloats about. Every year or so, we get a small hint about the platform's popularity, but no monthly active users or total sales units have been shared recently, or ever. This lack of marketing has lead many to think Android TV is close to abandoned, when in fact Google has been actively pushing the platform to vendors more than end users. In March, the company announced 160+ TV providers were using its OS. Now, we're getting another hint: YouTube for Android TV has just passed 50 million installs.
YouTube is one of a few pre-installed apps on any TV, set-top box, or projector running the platform, but it's also one of two that we can directly link to Android TV since its listing is separate from the regular YouTube Android app. (The second one is Chromecast Built In.) Other apps like the Play Store, Play Movies & TV, Play Music, and Play Games use the same listing for both platforms, so we don't know what portion of their total install numbers comes from Android TV and which portion is due to phones.
That's why YouTube's number is interesting. Since Google calculates how many unique accounts have installed the app, regardless of how many devices it was installed on (or reinstalled or updated), we can only say that 50 million different accounts have signed in to Android TV at one point in time. Some accounts may belong to the same person, other users may have abandoned the platform long ago, some people may have bought several TVs and set-top boxes over the years and signed into them with the same account, and some may have one TV linked to different household users. That's not to mention the same unit being sold and used second-hand by another account. Extrapolation from accounts to users to units is impossible in that context — even if these edge cases might cancel each other out — but it provides us with a ballpark number of Android TV's success and growth so far.
For the stat nerds among you, it's interesting to see the app's install numbers over the years, courtesy of Appbrain. It reached 1 million installs in 99 days, then took a little less than a year to go to 5 million. After that, the growth was relatively steady at ~2x year over year, in line with Google's Android TV growth claims: 5 to 10 million in 14 months, and 10 to 50 million in 27 months. It'd be interesting to see if the app keeps that same momentum going and doubles to 100 million installs in one year, or if it'll hit a market saturation cap soon.