How many super volcanoes are there in the world?

The Earth has approximately 20 known supervolcanoes, according to James Morgan, who write for the BBC News. What constitutes a "supervolcano" is not clearly defined, so the term is avoided in the literature, but it is often used in reference to two distinct phenomena.

According to Wikipedia, a supervolcano is any volcano that is capable of ejecting at least 1,000 cubic kilometers of rock in a single event. The term is used in connection with both large eruptions, such as occurred at Toba 78,000 years ago, and with large basaltic lava flows, such as the eruption of the Siberian traps.