There are about 550 volanoes on earth that have erupted in historic times. In addition, there are an equivalent number of dormant volcanoes that have not erupted in historic time, but have erupted in the past 10,000 years. On any given day, there are about ten volcanoes that are actively erupting.
Eruptions are highly variable in size and explosiveness.
As the table below demonstrates, small eruptions are more frequent than larger eruptions. It takes a greater amount of time to build up the necessary gas pressures needed for the larger eruptions. The global frequency of small eruptions, producing 0.001 to .01 cubic kilometers of volcanic material, is once every few months, whereas the frequency of very large eruptions, producing thousands of cubic kilometers of ash, is about once every 100,000 years. The last truly giant eruption on earth occurred at the Toba volcano on Sumatra 74,000 years ago. It is extremely difficult to asses the magnitude of an eruption in a truly quantitative way.