We have 12 zodiac signs, not because there are 12 constellations along the ecliptic, but rather for the same reason that we have 12 months: because there are 12 lunar cycles for each solar cycle.
In other words, the sun takes one year to travel once completely along the ecliptic path. During that time, the moon travels the ecliptic 12 times. Ancient astronomers in Babylon designated the 12 signs of astrology after meticulous observation of these cycles.
Some are quick to assume that the Ophiuchus constellation is actually a 13th zodiac sign, but they are wrong. They erroneously claim (without much knowledge of astrology) that since the Ophiuchus constellation is along the ecliptic, that Ophiuchus must be the 13th zodiac sign.
But, there are as many as 22 star constellations along the ecliptic band. The number of constellations does not change our zodiac signs. Our zodiac signs are calculated by the Earth’s solar seasons and lunar months, not by the number of arbitrarily-designated constellations. The zodiac signs are 30-degree sections of space along the ecliptic band.
The popular 12 star constellations, with their varying degrees and sizes, are not the actual zodiac signs. These 12 constellations were named after the zodiac signs they represent, not the other way around!
Image Credit: Sidney Hall [Public domain], via Wikimedia Commons