its 1 supernova per century per galaxy. there are many galaxies: more than 10 stars go supernova every second across the universe. tens of thousands have gone supernova since the article was posted to HN. tiny percentages in a large sample are huge numbers, you might even say 'astronomical'.
There is a really good intuitive analogy hiding here for the scale of the universe:
Our galaxy is to the observable universe as a tenth of a second is to your entire lifetime.