From The Verge:
The official Raspberry Pi magazine just announced that over 12.5 million of the affordable little Linux boards have been sold since the original Pi was launched in 2012. As The MagPipoints out, this puts the Raspberry Pi past Commodore 64 sales, according to some estimates. That would make the Pi the third best-selling "general purpose computer" ever, behind Apple Macintosh and Microsoft Windows PCs. As commenters have pointed out, this isn't a precisely fair comparison, because there were other Commodore models than just the 64, but it's still a nice milestone all the same.
It's interesting that the most sold is the latest, the RaspberryPi 3, which is in my mind the first one that really reached the potential of a single-board Linux computer (built-in Wifi and decent processing power). So that bodes well for continued accelerating growth.
By comparison, as of four years ago, the Arduino team had estimated that there were 1.4m Arduinos and clones sold and presumably it's several times that now (Arduino was launched in 2007, five years earlier than Raspberry Pi). The two don't really compete -- Arduino is best for physical computing and interfacing with sensor, while RPi is best for video and running Python code fast -- but I do find that I almost never use Arduinos anymore. There are enough good RPi "hats" that can read and control other hardware that I rarely find the need to add an Arduino into projects now.