There are 8 US Post Offices in Clay County serving a total of 6,883 residents equating to 860 residents per post office. It's estimated that approximately 10,986 packages are handled across the 8 county post offices annually. Clay County is a county in the U.S. state of Nebraska. As of the 2010 census, the population was 6,542. Its county seat is Clay Center. The county was formed in 1855 and later organized in 1871. It was named after Henry Clay, a member of the United States Senate from Kentucky and United States Secretary of State in the 19th century. In the Nebraska license plate system, Clay County is represented by the prefix 30 (it had the thirtieth-largest number of vehicles registered in the county when the license plate system was established in 1922)."