Tornado climatology
The United States averaged 1,274 tornadoes per year in the last decade. April 2011 saw the most tornadoes ever recorded for any month in the US National Weather Service's history, 875; the previous record was 542 in one month. It has more tornadoes yearly than any other country and reports more violent (F4 and F5) tornadoes than anywhere else.
Tornado climatology
Not every thunderstorm, supercell, squall line, or tropical cyclone will produce a tornado. Precisely the right atmospheric conditions are required for the formation of even a weak tornado. On the other hand, 700 or more tornadoes a year are reported in the contiguous United States.