How many supernovas have been discovered?
All supernovas known are classified as "SN" for supernova "YYYY" for year and "Z" for the number discovered in "YYYY". So SN1987A is a supernova discovered in 1987 and was the first one discovered that year. So any supernova discovery can be deduced from it's classification. See related link for a list of major supernova.
Elements have been discovered all over the world and some are named after places where they were discovered. Some such as iron, gold, silver, copper etc have been known since antiquity and nobody knows where they were discovered. Many of the elements beyond Uranium were discovered in the atomic age in the USA. Terbium and other lanthanides were discovered in Sweden and named after towns in that country.