There are 7 US Post Offices in Sumner County serving a total of 148,729 residents equating to 21,247 residents per post office. It's estimated that approximately 237,388 packages are handled across the 7 county post offices annually. Sumner County is a county located in the U.S. state of Tennessee. As of the 2010 census, the population was 160,645. Its county seat is Gallatin, and its largest city is Hendersonville. The county is named for American Revolutionary War hero General Jethro Sumner. Sumner County is part of the Nashville-Davidson\u2013Murfreesboro\u2013Franklin, TN Metropolitan Statistical Area."