The current version of the flag has fifty stars which represent the fifty states of the United States.
The original American flag had thirteen stars. As further states have been admitted to the union, extra stars have been added, but the number of stripes has remained at the original thirteen. The exception was the 15-star flag, which also had 15 stripes. It was this 15-star flag which inspired Francis Scott Key to write The Star-Spangled Banner.
When the flag is changed, the change always takes place on July 4, as a consequence of the Flag Act of 1818. The most recent change, from forty-nine stars to fifty, occurred in 1960, after Hawaii was granted statehood.