ZDNet is citing a source that claim Microsoft has sold at least 18 million Xbox One consoles.
Mary Jo Foley took to the Windows Weekly podcast (via GameSpot) to make the claim.
If true, that places the Xbox One at approximately half as many PS4s sold to date. Sony has recently said the PS4 has sold 35.9 million units as of early January.
Microsoft recently changed its reporting, and as such, will not confirm how many consoles are in homes.