To a first approximation... zero. That is to say, I don't know any (and I've looked a bit) and the realities of the limited hardware support make it difficult to imagine. I don't know of anybody using Inferno either, but I'm less familiar with it.
I'd also dispute the suggestion that Plan9 is more advanced than Linux. It's different and Plan9 definitely does some interesting things, but that doesn't automatically make it more advanced. Ignoring that "advanced" is hard to quantify, I'd more that Plan9 is simply based on different concepts. It's like comparing Windows and Linux, albeit maybe not quite so extreme as Plan9 and Linux have more in common and Plan9 is simply not as heavily developed as either OS.
Coraid used to build and sell systems based on Plan9, but I believe they don't exist anymore, though I don't mean to imply Plan9 was at fault, quite the contrary.
