50 (more or less)
As many of you have noticed, this question has a number of possible answers, depending on how one defines an "active" volcano. So far there appears to be no less than four "correct" answers.
The most frequently cited number on the Internet is 50, from a variety of sources quoting the Columbia Encyclopedia. The US Geological Survey currently lists 52 volcanoes for which there is a historical account of an eruption. However, a 1992 publication from the USGS titled "Living with Volcanoes" states that there are 65 "active and potentially active" (whatever that means) volcanoes in the US. The Smithsonian Institution also maintains data on volcanoes, of a variety of different classifications. (The Smithsonian publication "Volcanoes of the World" seems to have provided several different answers to this question.) There is any number of other answers as well, depending largely on which historical accounts are accepted and which are not.
All the sources agree that the US ranks third in the world for the number of active volcanoes, behind Japan and Indonesia.
WHO GOT IT RIGHT: George Oldroyd, Bob Milligan, Pete Daggett, Dr Feelgood, Rick Gough, Robbie Moore, and Marty Paul.