The Continental Congress specified thirteen stripes for the United States flag in its 1777 Flag Resolution.
Congress approved the resolution on June 14, 1777. It called for thirteen alternating red and white stripes and thirteen white stars on a blue field, representing the thirteen states or former colonies.
The number of stripes did not increase as new states joined the Union. In 1795, Congress changed the flag to fifteen stripes and fifteen stars after Vermont and Kentucky entered, but the design became unwieldy as more states were admitted. The Flag Act of 1818 restored thirteen stripes permanently while allowing the number of stars to grow.
The current flag has fifty stars, one for each state, but it still has thirteen stripes. The stripes therefore refer to the original thirteen states, not the present number of states.