Just started reading an article over at Ars Technica, concerning the M1 Mac Mini, and saw this:
"The Mac mini can only drive two displays at a time, and one of those has to be over HDMI. Since that HDMI port is HDMI 2.0, it doesn’t have the throughput to handle 4K at 120Hz, or 8K at 60Hz. There aren’t many monitors like that now, but there increasingly will be over the next few years."
So, basically, via HDMI, you can only run 4K @ 60Hz and not run 8K at all. I'm assuming the USB-C ports are not limited like this.
Hopefully this helps!