Windows will recognize a maximum of 8 GPUs installed on your computer, so you can have a maximum of 4 6990s, but you can only crossfire four of those GPUs. When you dont crossfire cards, they only work on rendering what is on the screen conncected to them, and you can specifically use them for GPGPU. Google search FASTRA. FASTRA II actually uses 13 GPUs, but requires a kernel and BIOS patch.

For your purposes, I'm assuming bitcoin mining, you don't actually need crossfire.