The correct answer is - 10 states.
The Mississippi River is one of the longest rivers in the world. It starts from the far north of the United States, close to the Canadian border, and in a roughly north-south manner it moves towards the Gulf of Mexico where it ends. Since this river cuts pretty much the entire country like a line from north to south, it is not hard to imagine that it runs through lot of states, or that it is used as a natural border for some of the states. The 10 states to which the Mississippi River forms part of their border are: Minnesota, Wisconsin, Iowa, Illinois, Missouri, Kentucky, Arkansas, Tennessee, Louisiana, and Mississippi.