I used to freak out somewhere between 40 and 50, but since that's been a fairly consistent number lately...
Well, anything above 70 starts to get me panicky that I'm moving wayyy too slow and should cut threads down. (Spoiler: I never end threads until they've gone their full course unless someone drops it first, regardless of how overwhelmed I get). Of course, I go into OH GOD I NEED TO DROP PEOPLE
...Until I clear my inbox and then I'm all pumped that I can handle anything. And considering I'm in something close to 130 threads right now, I think it's pretty nifty!