Recent rumours suggested that Windows 8 would reach a maximum of 8400 builds before it went RTM, that rumour has been squashed today as information from a source familiar with Microsoft's plans have revealed that Windows 8 has reached build 8500.
(http://i47.tinypic.com/2qc17p4.jpg)
The information comes from canouna, who says "It's legit, current builds from win8_rtm folder" in a reply from a user which is asking about the screenshot above. This means that Windows 8 has reached build 8500, and that RTM is getting closer.
It is currently unclear when or what build number the company will finalize Windows 8 at, we think Windows 8 will be completed by or on 8600, which would make 1000 builds more from Windows 7 RTM.
source: WinUnleaked