How many non-dormant volcanoes are there?

We posed this question to Chris Kilburn from University College London...

Chris K - There are about 600 to 1,000 volcanoes for which we have records of eruption, that have actually been observed, but using radioactive dating techniques we think there are about 10,000 that have been active in the last 10,000 years, and have the potential to erupt again.