Why are there 12 zodiac signs and not more or less?

The plane in which the earth orbits the sun is called the ecliptic. From earth, the sun appears to move across the ecliptic in one year. This circle of 360 degrees is divided into 12 equal sectors of 30 degrees each. Each of these segments is represented by one zodiac sign. The zodiac is the circle or belt, which the ancients said extends 8 degrees on either side of the ecliptic. The word zodiac literally means circle of animals. In the context of astrology, it refers to the animal symbols of the great circle around which the sun appears to move throughout the year.