From the Wikipedia article about US counties:
- Independent cities: These are cities that legally belong to no county. As of 2004, there are 42 such cities in the United States: Baltimore, Maryland; Carson City, Nevada; St. Louis, Missouri; and all 39 cities in Virginia, where any municipality incorporated as a city (in contrast to town) is by law severed from any county that might otherwise have contained it. ,,,
- A city and its containing county may be merged to form a consolidated city-county, which is considered both a city and a county under state law. Examples include Denver, Colorado; San Francisco, California; and Indianapolis, Indiana. ...
- A city may extend across county boundaries. Atlanta, Georgia; Columbus, Ohio; Kansas City, Missouri; Houston, Texas; Chicago, Illinois; and Oklahoma City, Oklahoma, are examples. The city of Dallas, Texas, is in parts of five counties, while New York City contains within itself five counties, each of which is coextensive with one of the five boroughs of the city
- Counties are used in 48 of the 50 states, while Louisiana is divided into parishes and Alaska into boroughs.
- The USA has 3,141 counties or county-equivalent administrative units
- The average U.S. county population is about 100,000, but only 16.7% of U.S. counties have more than 100,000 inhabitants.
- There are on average 62 counties per state.
- The least populated county is Loving County, Texas, with a population of 67.