There are folks running 9-10 card rigs, with undervolted cards, but you just don't hear about it nor see DIY threads because they have the difficulty in mind.
So if you don't see or hear about them, how do you know they exist? Did you dream them up?
One has to assume that every successful setup isn't shared on the site. You don't agree?
This community has been very open about sharing most of their trade secrets even, so I wouldn't agree.
8 card rigs exist, but only 2-3 people have the skills needed to make them, and even they have let the cat out of the bag on several occassions, you can run 8 cards on linux with minor tweaks.
I have not, in my year and a half on these forums, heard of a 10 card rig, so its too well kept a secret to be possible.
I did read that someone managed to get 10 cards detected in Linux a couple of days ago but that was a noob account and he hasnt shared any image or proof, so I do not trust that information yet.
Some enterprising members like Jstenfanop are developing 8 card modules that can be added in pairs to run a total of 16 cards off of essentially one system, but that's specialized hardware, and its still in production.