I read:-
"While there is, on average, only one supernova per galaxy per century, there is something on the order of 100 billion galaxies in the observable Universe. Taking 10 billion years for the age of the Universe (it's actually 13.7 billion, but stars didn't form for the first few hundred million), Dr. Richard Mushotzky of the NASA Goddard Space Flight Center, derived a figure of 1 billion supernovae per year, or
30 supernovae per second in the observable Universe!"
"......astronomers estimate only about one out of 100,000 supernovae produce a hypernova. This works out to about
one gamma-ray burst per day, which is in fact what is observed."
(That is presuming every hypernovae produces a GRB. I think this is still under investigation)
That is a lot of exploding stars!
(News from Dave Reneke's site -
http://www.davidreneke.com/astro-space-news )