I'm using an ASRock Z97 Extreme4 with 6 pcie lanes because I had it lying around and I don't seem to be having any problems with it. It has an M.2 port also so it might be able to do 7 but i'm not sure, I never tried it.
What cards/os/system are you running ? I have the exact same motherboard and it refuses to work with more than 4 gpus.