Given the population of South Dakota and publisher ratio in the United States, it would indicate more than 3000 JWs. Even if South Dakota is well below average for JWs, 330 is still far too low.
According to JW website there are five congregations in Rapid City alone, and seven in Sioux Falls. Then there's Pierre and Chamberlain as mentioned, and Gregory and Watertown and other rural congregations. Definitely a lot more than 330 JWs in South Dakota.