The first US flag had 13 stars and 13 stripes, which represented the 13 original states. When Vermont and Kentucky entered the Union, two additional stripes and two additional stars appeared on the flag. As other new states were joining, no new stars or stripes were added to avoid the mess. In 1818 it was decided to return to 13 stripes to represent the original colonies. Each star began symbolizing one state. That's why the number of stripes remained the same since then. So, on the today's flag we see 50 stars as the symbols of 50 states.

Thus, the right answer to the question is 15.

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