I have personally verified that you can not install more than 6 GPUs with windows 7.
The only thing you've personally verified is the fact that you were unable to install more than 6 GPUs with windows 7.
Windows XP 32 bit + 8 GPUs -
http://foldingforum.org/viewtopic.php?f=38&t=7902&start=270#p115136Yeah.. that's two core nvidia cards, but they do count as GPUs, right ; )
I think that there is no hard coded limit or any limitation of GPUs in both Linux and Windows.
However when installing 8+ GPUs there are certain issues with BIOS unable to map all the GPUs.
Can't remember the project name right now, but guys made 10(not sure about the number but above 8 ) nvidia GPU rig, they had to use custom BIOS and custom linux kernel pathes to make it work.
Plus there is known issue with Windows ATI driver, which prevents you from n+ GPUs drivers being properly installed.
Some people claim n=4, some n=5.
For my setup(GA-990FXA-UD3 + 2x6950 + 2x7950) n=5. And in order to use 6GPUs in Windows I needed either to use 12.6 driver downloaded from AMD or 13.1 driver downloaded from techpowerup.
Also that 13.1 from techpowerup driver is different from 13.1 from AMD driver only in registry, so I assume that with some mod it is possible to use any driver version for 6GPU rigs.
So unless s1 explains why it is not possible technically to use 6+ AMD GPUs in Windows I consider it is doable.
"Windows sux"/"I was not able to use more than n GPUs in Windows"/"AMD drivers are crap" are not technical explanations.