> But we have to consider the speed of light. So what we are really asking is something like what is the probability that light will arrive at Earth in some interval from a supernova.

We really don't. (We've already largely accounted for it, by only counting the stars in the observable universe.)

So yes, stars supernova at a rate of ~30/second, from our current frame of reference. However, many of them are occluded by dust, or are too faint to differentiate from their neighbours/parent galaxies, or are too red-shifted away...