These notes are from my attempt to get over 16 GPUs working in an ASRock Rack 4U8G-F/C621, despite only having space for 10 GPUs I had 8 Quadro M10s which have 4 GPUs each. I tried to get all 8 cards/32GPUs working but I was running into issues with PCIe device enumeration, I found that the system would only boot with 6 cards/24GPUs installed.
sudo nano /etc/default/grub
# add kernel parameter (in GRUB_CMDLINE_LINUX) to the end: pci=realloc
sudo update-grubfor RHEL: