There are 11 US Post Offices in Sioux County serving a total of 33,256 residents equating to 3,023 residents per post office. It's estimated that approximately 53,080 packages are handled across the 11 county post offices annually. Sioux County is a county located in the U.S. state of Iowa. As of the 2010 census, the population was 33,704. Its county seat is Orange City.