I believe the classification varies from state to state. You could have a place of 500,000 people in one state but it would be called a town , then in another state you could have a place of 1,000 people and it would be called a city .So it depends on the legislation of each state, and also people s general perception of a city .
thats not a city. a city is a population of at least 100,000 by the census. im not sure how the other municipalities or whatever are classified though. there are so many cities because so many people live there !
Read up on Central Place Theory in Wikipedia or elsewhere.