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.