There are about 1,350 potentially active volcanoes worldwide, aside from the continuous belts of volcanoes on the ocean floor at spreading centers like the Mid-Atlantic Ridge. About 500 of those 1,350 volcanoes have erupted in historical time. Many of those are located along the Pacific Rim in what is known as the "Ring of Fire." In the United States, volcanoes in the Cascade Range and Alaska (Aleutian volcanic chain) are part of the Ring, while Hawaiian volcanoes form over a 'hot spot' near the center of the Ring.
There are about 170 potentially active volcanoes in the United States and its territories.
The U.S. Geological Survey assesses and monitors hazards at volcanoes within the United States and its territories. Good sources for information about volcanoes outside the United States include Oregon State University's Volcano World and the Smithsonian Institution's Global Volcanism Program.