Mac OS only supports Single Stream Transport (SST) meaning it can only support one extended display over a single USB-C connection. The 2nd display will a mirror of the 1st display.
Windows OS supports Multi-Stream Transport (MST) on supported computers and can support 3 extended displays over a single USB-C connection.
If one extended display is connected, the max resolution is 4K60Hz (on devices that support DisplayPort 1.4).
If two extended displays are connected, the max resolution is 2x4K30Hz.
If three extended displays (2xHDMI+DP or 2xHDMI+VGA) are connected, max resolution are 2x4K30Hz+720p or 2x1080p+4K30Hz.
For mirrored displays, max resolution is 4K60Hz are each mirrored display.