@ david rogger - First welcome to the Resolume forum.
Yes, you're right, you can connect 6 outputs to a lot of AMD GPUs (including the D300/500/700 are used in Mac Pros) but that still only gives you 5 outputs for projectors/displays since you must use one output for the user interface.
As far as I understand, the only GPU out there that "works" with Resolume that has more than 6 outputs natively is the Nvidia NVS 810. Take a look at this thread
Since Arena 5 supports outputting to capture cards, the new BMD DeckLink Quad 2
would allow to output to 8 displays since any channel can be programmed as input or output.
As soon as I can get my hands on one of these cars (they are not commercially available yet) I will test and report but the one catch though is that it's broadcast gear so it does NOT support computer resolution just VESA resolution. That means you'd be limited to 1080HD ,720HD, NTSC and PAL so you couldn't output XGA or UXGA for instance.