There are 21 US Post Offices in Greenville County serving a total of 254,041 residents equating to 12,097 residents per post office. It's estimated that approximately 405,478 packages are handled across the 21 county post offices annually. Greenville County is a county located in the state of South Carolina, in the United States. As of the 2010 census, the population was 451,225, making it the most populous county in the state. Its county seat is Greenville. Greenville County is included in the Greenville-Anderson-Mauldin, SC Metropolitan Statistical Area.