I’m running Ubuntu 16.04 and I have good success with 4 cards, with nothing additional on my kernel command line.
The motherboard is an Asus Z270-A motherboard with the PCI Express setting currently set to Gen2 instead of Gen3.
However when I added two additional cards I encountered an I/O region error in dmesg from the nvidia module (version 384.98) and i basically had to start adding in an acpi=off command line option for the kernel, to get six cards to start working.
Currently the relevant line in my /etc/default/grub looks like this:
GRUB_CMDLINE_LINUX=“noapic pci=noacpi,biosirq acpi=off vga=788”
And my dmesg for nvidia shows an RmInitAdapter failed! (0x24:0xf:1077) message.
The EWBF miner (version 0.3.4b) shows reduced Sol/s for each card when running all of them, or 0 Sol/s particularly after a GPU0 Rejected share message came up.
I will be looking for a bios update over the holiday to see if that fixes anything. It would be nice if the system didn’t need the acpi=off line because the system runs fine with enough address space for 4 cards, but it looks like if I go for any additional I will have to tweak something.