Number Of Volcanoes In The U.S.

Dr. Charles Wood, the head of VolcanoWorld, and Jurgen Kienely of the University of Alaska published a book in 1990 called "Volcanoes of North America." They list 262 volcanoes. If you subtract out those in Canada and those in Hawai'i (not really part of North America anyway) you come up with 224 volcanoes in the continental USA + Alaska.
By far, most of those volcanoes are in Alaska. There are 116 volcanoes listed in the Alaska section.
If you want some more details, you might want to take a look for this book in your library.