Hallo,
I'm struggling with multiple 4k displays connected to an AMD Asus Mini PC PN50 (4500u cpu) barebone with 1 DP1.4, 1 HDMI 2.0, 2xUSB-3.2 gen 2 with Displayport support.
I'm able to connect 1 display via HDMI 2.0, one display via Displayport 1.4, and via adapter, one usb-3 gen3.2 to an HDMI 2.0 monitor (three in common).
I can see and adopt the changes in 'display (monitor) settings in KDE settings and accept changes. Everything fine.
But after reboot, one display is dark and I can not login into SDDM screen on another monitor. It is not possible to enter any sign or letter. Seems, SDDM doesn't know about system's display settings.
With one or two monitors I can login without problems.

So, how can I tell SDDM login screen (greeter) to accept any letter to login?
My configuration:
PN50 (ASUS) with AMD 4500U cpu and built-in gpu.
ARCH Linux, Kernel 5.10.10
KDE Plasma 5.20.5
qt version 5.15.2
I also have installed evdi kernel modules (AUR) and displaylinkmanager service is loaded and active.

And, pls, does anybody really know, how many displays can be connected, e.g. I'd have a pc with a dedicated graphics adapter (e.g. AMD RX5000 with 3 DP and 1 HDMI)? Is it possible to make my ASUS PN50 run with the build-in DP 1.4, HDMI 2.0, 2xUSB-3 gen 3.2 with Displayport support?
On the merchandising website of ASUS they offered 4 4k monitors to be able to be connected at the same time at Windows OS.
https://www.asus.com/Displays-Desktops/Mini-PCs/All-series/Mini-PC-PN50/

Then you very much in advance.

Linuxrp