I got two rigs, one with z170 mobo and another one ga-p67a-udr3-b2. Eventually the z170 works only with 5 GPUs no matter how hard I tried. And the ancient mobo p67(many may never heard of it) works with 6 perfectly even though it has only 5 pcie slots(using 1->3 splitter). Will try to add more once I got more cards. Will let you know the result.
BTW, heard that mobo limits of number of GPU is 8. Any one tried more than that?