Brendan Sinclair wrote:

In the months following the Xbox One's November 2013 launch, Sony and Microsoft periodically shared sales figures for PS4 and Xbox One, respectively. However, it soon became clear that Sony had taken a solid lead in the generation's early days, and Microsoft got very quiet about Xbox One sales. The last official word the company gave out was nearly 10 million shipped just before the system's first anniversary. After that, it wrapped 360 sales in with Xbox One sales in its quarterly reports, then abandoned that entirely in favor of Xbox Live engagement figures.

Late last week, reports circulated suggesting that the Xbox One's installed base was about 18 million systems. That stemmed from the latest episode of Windows Weekly (around the 9-minute mark), in which tech journalist Mary Jo Foley discussed Microsoft's recent announcement that Windows 10 had been active on 200 million devices in the last month. In breaking down that number between PCs, tablets, smartphones, and Xbox Ones, Foley said, "I can tell you what one source told me and I can't verify this," before sharing that 18 million of those active devices were supposedly Xbox Ones.