There are 10 US Post Offices in Shelby County serving a total of 19,915 residents equating to 1,992 residents per post office. It's estimated that approximately 31,787 packages are handled across the 10 county post offices annually. Shelby County is a county located in the U.S. state of Illinois. According to the 2010 census, it had a population of 22,363. Its county seat is Shelbyville.