For controlling a rack mixer via UC Surface on a computer, the mixer needs to be connected via USB. Two mixers can be connected via one Ethernet cable (CAT5e or CAT6) and controlled by a computer (USB) or remote control (tablet, router). Remote Control is ultimately needed when the second mixer is used in Monitor Mixer Mode or Stand-Alone Mode.
For connecting more than two mixers, you will need the SW5E AVB switch. You should be aware that using several 16R does not increase channel count. Because of its limited DSP power there will never be more than 16 input channels, 6 flex mixes and 2 effect slots. The two mentioned "stagebox modes" will allow the single use of rack mixers, but all network mixers will always share their inputs.