How Many Supernovas Have We Been Seeing?

In the 400 years since Kepler's observation, we should have seen about 20 supernovas in the Milky Way. So that'd be one every 200 years, right? Not one every 400 years. We don't think that the historical record is complete. And also we don't think people have seen all the supernovas that were out there. But something we'll dig into in a minute is how often we expect these things to happen and how likely we are to see one if it does happen in our galaxy.