Would be interested to know too. Just checked and the APP says that 100,000+ have been installed (far from an accurate number)
And then how many get returned (getting close to considering it myself)
@JesseJ according to this article on 22 November there seemed to be 175.000 editions being sold. Some of them might also be Premiere editions. This number are the downloads from the Stadia App, therefore it's not a 100% reliable source for the sales, but at least it's something.
I definetely believe though that it's less than other platforms, because most people are still waiting and checking if it's worth to buy and use Stadia, because most of them have a functional PS4, XboX or PC at thome. And as long as Stadia can't proof to be better, I believe those will not switch to Stadia anyway.
I could imagine though, that if more games are available on Stadia and when some major Bugs/glitches are fixed, it could be even more interesting and then the sales could be bigger as well.