asus P6T7 WS SuperComputer + 5x gtx 970 not recognize in device tap

asus P6T7 WS SuperComputer motherboard with 2x nForce 200 means its capable 7 gpus.
my early system 6x gtx590 + 1x quadro 7000.
it means 13 gpu in system.
worked properly …

I upgrade my gpus Gtx 970 series…

4x gtx 970 work perfectly but more than 4 gpu not install the driver problem.
lastest motherboard bios insstaled.
exclamation marks on gpus at system device tap
system tries to regonize :black screen then desktop few times but aftter its show exclamation marks.
system not recognize any the gpus.

I tried differeent combination diffferent slot and brand of gtx 970
there is a way to make my system work more than 4 gtx 970

if its imposible

4x gtx 970 + 3x(gtx 980 or 980ti or gtx 1070 (different than gtx 970))
works properly ?