I have that board. I have it run it with as many as 5 cards before. It was tricky to get setup.
What worked for me was having a 16x->16x extender on the main pcie slot. After that the others could be 1x-16x either powered or not.
One thing to note is you will have to do the pcie1x jumper trick to get the board to see all the 1x extended GPUs.
Right now I have 3 5970s on it and it runs great. It won't work on BAMT though without help.
Let me know how you get on.