At an Elder Scrolls Online press event, Matt Firor confirmed to our own Bill Murphy that the game is sporting 8.5 million players. At E3 2016, ESO had 7 million players and has grown by 1.5 million since then. This number is based off of units sold and is not simply a tally of registered accounts.
According to Firor, the population is split fairly evenly among all platforms. Currently, Elder Scrolls Online is available on PC, PlayStation 4 and XBox One.
Congratulations, Elder Scrolls Online.