> I also vote +1 for returning None when the information is unknown.

I still don't like it.
If a function returns a number of CPU, it should either return an
integer >= 1, or raise an exception.
None is *not* an integer.

And returning an exception is IMO useles, since the user can't do
anything with anyway, other than fallback to 1.

> Just write "os.cpu_count() or 1" if you need 1 when the count is unknown ;-)

os.cpu_count() or 1 is an ugly idiom.

> See also #17444, Trent Nelson wrote an implementation of os.cpu_count().

I don't see exactly what this C implementation brings over the one in
multiprocessing (which is written in Python)?