There are 9 US Post Offices in Plymouth County serving a total of 23,504 residents equating to 2,612 residents per post office. It's estimated that approximately 37,515 packages are handled across the 9 county post offices annually. Plymouth County is a county located in the U.S. state of Iowa. As of the 2010 census, the population was 24,986. The county seat is Le Mars. Plymouth County was named after Plymouth, Massachusetts. Plymouth County is part of the Sioux City, IA-NE-SD Metropolitan Statistical Area."