It's been around six weeks since the game debuted.
Diablo IV was rewarded with both great reviews and a matching commercial success at launch, and it seems like the success train has kept on rolling since then. In Activision Blizzard's latest financial earnings report, the company reveals that the game has climbed over ten million players already.
As the game isn't a part of Game Pass (or any similar subscription service), we assume the number isn't too far from the actual amount of copies the game has sold. And speaking of Game Pass, Blizzard recently said that the game would not be included with Game Pass. While this is something that certainly could (and possibly also will) change when/if Microsoft acquire Activision Blizzard, it won't be until late this year as the deadline for the merger was just postponed until October.