There isn't a constellation called "Plow." You're likely thinking of Ursa Major, which is also known as The Great Bear.

The Big Dipper is an asterism, which is a pattern of stars within a constellation. It is part of the constellation Ursa Major. It is commonly called "The Plow" in some regions.

The Big Dipper itself doesn't have a set number of stars as it is a pattern. It's made up of seven bright stars that are easily visible. However, the constellation Ursa Major as a whole has many more stars than the Big Dipper.