The answer depends on where you find the information.
The NOAA website lists an average of 28 tornadoes per year for South Dakota.
The US Tornadoes website list an average of 36 tornadoes per year for South Dakota.
That is difficult to determine. Official records only go back as far as 1950, and many weak tornadoes in the earlier part of this period were missed. The state's sparse population means that many tornadoes go unseen and undocumented.
With that said, since 1950 there have been 1,688 recorded tornadoes in South Dakota.
South Dakota averages about 35 tornadoes per year.
It depends on which tornado you are referring to. South Dakota has had many tornadoes. Most have not been killers but some have been. So far in 2010, however, the have been no tornado fatalities in South Dakota.
The greatest number of tornadoes occurs in Tornado Alley, a region in the central U.S. that stretches from Texas to South Dakota and Iowa.
There were 1,103 tornadoes in the U.S. in 2006.
There were 94 tornadoes in Alabama in 2008.
There were 105 tornadoes in Iowa in 2008.
15 tornadoes occurred in Michigan in 2011.
There were 20 tornadoes in Ohio in 2014.
There were 16 confirmed tornadoes in Arkansas in 2007.
There were 1,817 tornadoes recorded in the U.S. in 2004.
The U.S. had 1075 confirmed tornadoes in 2000.
There were 1374 tornadoes confirmed in the United States in 2003.