On the average very little. Even volcanoes that would be called active may not have erupted for hundreds of years, and dormant or extinct volcanoes may not have erupted in modern times - where modern refers to the past ten thousand years.

The more volcanoes you include in your average, the lower the answer will be. By some estimates there are more than a million volcanoes under the Ocean, which we can neither count, nor measure their emissions. Its also hard to decide how to count several eruption centers related to the same magma chamber.
Source(s):
http://en.wikipedia.org/wiki/Volcano#Scientific_classification_of_volcanoes
http://www.volcanodiscovery.com/en/volcanoes/faq/how_many_volcanoes.html

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