There are 22 US Post Offices in Polk County serving a total of 285,249 residents equating to 12,966 residents per post office. It's estimated that approximately 455,289 packages are handled across the 22 county post offices annually. Polk County is a county located in the U.S. state of Iowa. As of the 2010 census, the population was 430,640, making it the most populous county in Iowa. The county seat is Des Moines, which is also the capital city of Iowa. Polk County is included the Des Moines\u2013West Des Moines, IA Metropolitan Statistical Area.