I seem to remember this was a big point of contention when threaded Apache (vs just forking a billion processes) appeared - that if you went from 20 processes to 4 processes of 5 threads each you could hit the ulimit.

But ... that's a bad memory from long ago and far away.

If you're old enough to remember those days, then you're also old enough to start forgetting them. I speak for myself of course :)