621
xkcd #2878: Supernova
(imgs.xkcd.com)
Alt text:
They're a little cagey about exactly where the crossover point lies relative to the likelihood of devastating effects on the planet.
Wikipedia (Near-Earth Supernova) says that a 25 ly away supernova would wipe out half the ozone layer so that's probably the lower bound for what we want
Geez, how many stars do we have that close to us?
None. Space is big, and stars that can potentially supernova are rare. https://en.m.wikipedia.org/wiki/List_of_supernova_candidates
Your answer is a little misleading. I think you meant there are several stars that close to us, but none that can go supernova.
That's disappointing
So far, we know of 131 ~~stars~~ objects
https://en.wikipedia.org/wiki/List_of_nearest_stars_and_brown_dwarfs