There are 29 US Post Offices in Hillsborough County serving a total of 325,544 residents equating to 11,226 residents per post office. It's estimated that approximately 519,605 packages are handled across the 29 county post offices annually. Hillsborough County is the most populous county in the U.S. state of New Hampshire. As of the 2010 census, the population was 400,721. Its county seats are Manchester and Nashua. Hillsborough is northern New England's most populous county as well as its most densely populated. Hillsborough County comprises the Manchester-Nashua, NH Metropolitan Statistical Area, which is also included in the Boston-Worcester-Providence, MA-RI-NH-CT Combined Statistical Area."