Remark
Not many per square mile. Sounds like an ideal place to live.
Step One
Find the number of Square Miles.
Area = Height * Width
Area = 380 * 200
Area = 76,000 square miles.
Step Two
Find the Density
Formula
Density = Population / Square Miles
Population = 814,000 people
Square miles = 76000
Solve
Density = 814000/76000 = 10.7 people per square mile.
Remark
Isn't that incredible. It means if all the Area of South Dakota were given to the people living there, each family of 4 would get roughly 1/2 a square mile.