With https://github.com/RIOT-OS/RIOT/pull/14224 the idle thread became optional, so the main thread can have either 1 or 2 as PID, depending if the idle thread is included or not. As this might also change in the future, it is probably the best to just expect any number.
Overview
WARNING: RIOT's MicroPython port is currently quite incomplete!
This application provides an example on how to use MicroPython with RIOT. Please see the documentation of pkg/micropython.