There are NO such limitations; nvOC is designed to max of 13 cards without any modification of 1bash & 3main (can add more by modifying these two files). Try to verify the risers (swap them around with the working ones).
Hash rates dropped with your 8 cards??
OK, I think same - no limitations...
I've rate ~6400 Sol/s with 8 cards.
After connection next 3 cards, rate dropped to ~5100 Sol/s only. nvOS didn't detect more than 8 cards, as you can saw on nvidia-smi.
Well, risers maybe problem - I'm novice in mining, so I ordered more types. I using version v006C and v008S. On cards plugged on v006C blinking LEDs below power connectors on card. I've ordered 13 pcs of v008S (these:
www.aliexpress.com/item/TISHRIC-2018-Hot-VER008S-Molex-4PIN-15PIN-SATA-6PIN-3-in-1-PCIE-PCI-Express-Riser/32830747032.html). I hope that will works well.
Many thanks :-)