How many GPUs does Accent Password Recovery/Passcovery Suite supports?

For GPGPU you don't need to link GPUs together, actually it can even brings another compatibility problems. The main limiting factors here are - number of PCI-E slots, size of videocards and drivers.

NVIDIA supports up to 8 GPUs per system (and there were reports about more GPUs with modified BIOS). Situation with ATI GPUs is way more complex, right now even 2nd core of 5970 isn't officially supported with ATI Stream SDK. However there were several reports about working 2x5970 configs. Situation with 5+ ATI GPUs is really unclear now -- there were reports that 3x5970 working under Linux but none reports about Windows so hard to say how such system will act. And chances pretty high that system with 4x5970 will simply lock up during booting.

However, if you'll able to boot system and get all GPUs recognized by Windows our software will have no problems to use all of them (with proper license type).