There are 4 US Post Offices in Barnwell County serving a total of 23,521 residents equating to 5,880 residents per post office. It's estimated that approximately 37,542 packages are handled across the 4 county post offices annually. Barnwell County is a county located in the U.S. state of South Carolina. As of the 2010 census, its population was 22,621. Its county seat is Barnwell.