Answer:
Because there is no well-defined minimum size for a "supervolcano,"
there is no exact number of such volcanoes. Examples of
volcanoes that produced exceedingly voluminous pyroclastic
eruptions and formed large calderas in the past 2 million years
would include Yellowstone, Long Valley in eastern California,
Toba in Indonesia, and Taupo in New Zealand. Other "supervolcanoes"
would likely include the large caldera volcanoes of Japan,
Indonesia, Alaska (e.g. Aniakchak, Emmons, Fisher), and other
areas.
