What is the maximum value of max_workers that can be given in ThreadPoolExecutor?

Code :

def download_all_sites(sites):
with concurrent.futures.ThreadPoolExecutor(max_workers=500000) as executor:
    executor.map(download_site, sites)

Question is about threading in python. In the above code, what is the maximum value that can be given to max_workers?

I tried max_workers=50, 500, and after 5000 it seems to have no effect.

Can anyone answer this please?