MicroPython supports[1] PIC16 and SAMD21/SAMD51, STM32, ESP8266/ESP32 and more, but it also supports Zephyr as a target, and with it the platforms Zephyr supports[2].

So yeah not everything under the sun, but certainly not what I'd consider a "very small" number of MCUs.

Of course, support level varies among the platforms, but you're not going to be doing too fancy things in MicroPython I imagine.

[1]: https://github.com/micropython/micropython?tab=readme-ov-fil...

[2]: https://docs.zephyrproject.org/latest/boards/index.html