There are 2 quarts in a half-gallon. A gallon contains a total of 4 quarts, whether you are measuring volume using the U.S. or British Imperial System of measurements.
While there are two quarts in a half-gallon under both the U.S. and the British Imperial measurement systems, the actual volume contained in a half-gallon is slightly different under each measurement system. A U.S. half-gallon is 1.89 liters, while the imperial half-gallon is 2.27 liters. In the U.S., milk is often sold in a half-gallon plastic jug, while in the U.K., the closest equivalent container is most likely the 2-liter bottle.