you can get 6 cards to work on that board using 1x risers, shorting slot #5 and not using slot 4. no cards filling up the whole slot or 16x risers.
of course that depends on your OS. But 5 will work without issue if you use slots 1,2,3,6,7. Slot 5 is to be used last for the 6th card.
Not the thunderbolt version. Everything I was able to find on google is that most gaveup on this board for more than 3 gpu.