People don't want to reveal the secret to getting more than 7 gpus working. It gives them an edge that they don't want to share. I remember reading that the motherboard's bios needs to be modded for 8+ to work.
For what it's worth, I had a go at 8 and 9 gpus on an MSI Z87. Board would boot woth 8 but only detect 7. With 9, the board just wouldn't boot.
i think win10 works with 8gpu without any mods - just rumor, need to confirm.
I think it would work with 8, but the issue is getting the motherboard to work with 8.