Hmm…

So that's cool, but now I'm thinking: the distant galaxies are redshifted and time-dilated in equal proportion, and also more densly packed because the universe was smaller in the past, so I expect the actual rate of supernovas to be significantly smaller than simply multiplying 1/century/galaxy by 1e11 galaxies.

Edit: also I don't know if rate of supernovas changes over history thanks to different steller environments giving the population-1/2/3 generations of stars…