There are 5 US Post Offices in Boone County serving a total of 51,941 residents equating to 10,388 residents per post office. It's estimated that approximately 82,904 packages are handled across the 5 county post offices annually. Boone County is a county located in the U.S. state of Illinois.As of the 2010 census, the population was 54,165. Its county seat is Belvidere. Boone County is included in the Rockford, IL Metropolitan Statistical Area, as well as the Chicago metropolitan area."