1500 active volcanoes.
Extract from Volcano Discovery:
"There are probably millions of volcanoes that have been active during the whole lifespan of the earth. During the past 10,000 years, there are about 1500 volcanoes on land that are known to have been active, while the even larger number of submarine volcanoes is unknown."
"At present, there are about 600 volcanoes that have had known eruptions during recorded history, while about 50-70 volcanoes are active (erupting) each year..."