The mpremote help says you can use the following shortcuts:

  a0          connect to serial port "/dev/ttyACM0"
  a1          connect to serial port "/dev/ttyACM1"
  a2          connect to serial port "/dev/ttyACM2"
  a3          connect to serial port "/dev/ttyACM3"

  c0          connect to serial port "COM0"
  c1          connect to serial port "COM1"
  c2          connect to serial port "COM2"
  c3          connect to serial port "COM3"

  u0          connect to serial port "/dev/ttyUSB0"
  u1          connect to serial port "/dev/ttyUSB1"
  u2          connect to serial port "/dev/ttyUSB2"
  u3          connect to serial port "/dev/ttyUSB3"

That's good, but why limit them to 4? I had 5 devices connected so for the first 4 I could address easily, e.g. mpremote u3 but the 5th device I had to specify the full device path (because mpremote u4 is not accepted), i.e. I had to type mpremote connect /dev/ttyUSB4. Just seems like an unnecessary constraint and surely would be easy to fix.

This is using mpremote from master branch as of today (v1.20.0-50-g786013d46) .