[5./JG27.Farber]

How many active volcanoes in the world?

The answer to this common question depends upon use of the word "active." At least 20 volcanoes will probably be erupting as you read these words (Italy's Stromboli, for example, has been erupting for more than a thousand years); roughly 60 erupted each year through the 1990s; 154 in the full decade 1990-1999 (doesnt add up - Farber); about 550 have had historically documented eruptions; about 1300 (and perhaps more than 1500) have erupted in the Holocene (past 10,000 years); and some estimates of young seafloor volcanoes exceed a million. Because dormant intervals between major eruptions at a single volcano may last hundreds to thousands of years, dwarfing the relatively short historical record in many regions, it is misleading to restrict usage of "active volcano" to recorded human memories: we prefer to add another identifying word (e.g. "historically active" or "Holocene volcano").

The definition of "volcano" is as important in answering the number question as the definition of "active." Usage has varied widely, with "volcano" applied to individual vents, measured in meters, through volcanic edifices measured in tens of kilometers, to volcanic fields measured in hundreds of kilometers. We have tended toward the broader definition in our compilations, allowing the record of a single large plumbing system to be viewed as a whole, but this approach often requires careful work in field and laboratory to establish the integrity of a group's common magmatic link. The problem is particularly difficult in Iceland, where eruptions separated by many tens of kilometers along a single rift may share the same magmatic system. A "volcanic field," such as Mexico's Michoacán-Guanajuato field (comprising nearly 1,400 cinder cones, maars, and shield volcanoes derived from a single magmatic system, dotting a 200 x 250 km area) may be counted the same as a single volcanic edifice. Perhaps the most honest answer to the number question is that we do not really have an accurate count of the world's volcanoes, but that there are at least a thousand identified magma systems--on land alone--likely to erupt in the future.
How many active volcanoes known?
Erupting now: perhaps 20
Each year: 50-70
Each decade: about 160
Historical eruptions: about 550
Known Holocene eruptions (last 10,000 years): about 1300
Known (and possible) Holocene eruptions: about 1500

From some website - http://www.volcano.si.edu/faq/index.cfm?faq=03

The only thing I can think of is when they say each year they meean a vent on a volcano and when the say each decade they mean a magmatically connected system having eruptions from any of its vent - hence the number is smaller. However I am only guessing. Also I speculate the reason 20 are errupting "as we read" is because some never really stop, like - Stromboli