https://stackoverflow.com/questions/48970857/maximum-limit-on-number-of-threads-in-python

OSError: core1 in use.rppicow.py

import _thread
import time


def mythread():
    time.sleep(1000)

def main():
    threads = 0     #thread counter
    y = 1000000     #a MILLION of 'em!
    for i in range(y):
        try:
            x = _thread.start_new_thread(mythread, ())
            threads += 1    #thread counter
        except RuntimeError:    #too many throws a RuntimeError
            break
    print("{} threads created.\n".format(threads))

works.py

import _thread
import time


def mythread():
    time.sleep(10)
    _thread.exit()

_thread.start_new_thread(mythread, ())
# sleep 10s
# _thread.start_new_thread(mythread, ())

https://forum.micropython.org/viewtopic.php?t=11433&p=62551