There are 24 US Post Offices in Waukesha County serving a total of 306,376 residents equating to 12,766 residents per post office. It's estimated that approximately 489,010 packages are handled across the 24 county post offices annually. Waukesha County (pronounced WAW-ke-shaw) is a county in the U.S. state of Wisconsin. As of the 2010 census, the population was 389,891, making it the third-most populous county in Wisconsin. Its county seat is Waukesha. Waukesha County is included in the Milwaukee\u2013Waukesha\u2013West Allis, WI Metropolitan Statistical Area."