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Found this website where someone was using a RX 580 with Ubuntu 18.04 for two monitors. Needed to add a line to get both to work in Ubuntu: drivers - Dual Monitors on 18.04 with Radeon RX 580 GPU - Ask Ubuntu
You, sir. are legendary! Thank you! Please add an answer and I will accept it. Would you be able to elaborate as to why this works? Just so I can at least try and understand what's happening - always found configuring displays a nightmare sometimes with Ubuntu. – Zakalwe
I was on Ubuntu 14.04LTS with 2 graphic cards (amd firepro W600) and 12 screens on one desktop (6 screens on each graphic card, resolution 1920x1080).
But impossible to display more than 8 screens on freshly installed ubuntu 18.04 using auto detection. "xrandr" command display every screen but the maximum total size is 16384x16384
I tried to copy xorg.conf file in "/etc/X11/xorg.conf" changing the driver name to "amdgpu", disabling Xinerama (because it doesn't start with it) and setting the second screen to position 1 to do some tests but just 6 screens are enabled.
Do you have any idea how to display more than 8 screens on Ubuntu 18.04LTS ?
EDIT: Forgot to mention that this Linux Forum is a good place to find answers to your post: https://www.phoronix.com/forums/