There are 5 US Post Offices in Cheyenne County serving a total of 10,109 residents equating to 2,022 residents per post office. It's estimated that approximately 16,135 packages are handled across the 5 county post offices annually. Cheyenne County is a county in the U.S. state of Nebraska. As of the 2010 census, the population was 9,998. Its county seat is Sidney. The county was formed in 1871 and named after the Cheyenne Native American tribe. In the Nebraska license plate system, Cheyenne County is represented by the prefix 39 as it had the thirty-ninth-largest number of vehicles registered in the county when the license plate system was established in 1922."