It is hard to put an exact figure on the number of extinct volcanoes in the world; according to Oregon State University, the expectation that a volcano will never erupt again is no guarantee that this is actually the case. When it comes to predicting volcanic behavior, there is a lot of potential room for human error.
A volcano is classified as dormant if it has not been known to erupt in the past 10,000 years. This classification switches to "extinct" if scientists don't expect the volcano to erupt again, though there have been known eruptions from volcanoes that were once extinct. Complicating the possibility of an exact number of extinct volcanoes is the fact that there are many unknown volcanoes under the ocean.