I have an old Macbook from 2013 that runs Linux now with an Intel integrated GPU and it supports dual 4K monitors at 30Hz. I use the Thunderbolt ports as Mini DisplayPort outputs and connect to two 4K monitors with DisplayPort and it works fine.
The mini can support two external displays. One via thunderbolt at that same 6k and 60Hz and the other a 4k at 60Hz through the HDMI port.
I use the Dell D6000 and I run three (1080p) external monitors in addition to the built in monitor.
I have two Thunderbolt/USB-C monitors that I was hoping to daisy chain with one wire from my Mac. Alas it's not possible.
My hope is power into a dock. Thunderbolt from dock to laptop to power laptop. Thunderbolt/USB-C from dock into first monitor. Second Thunderbolt/USB-C from dock using the DisplayLink tech to second monitor.