When we think about the zodiac signs, the most common idea is that they are different personalities/a set of traits, based on the images of the constellations. We look at the representation of the constellations to understand their story and find a description for each archetype (ex: the lion, the maiden, the scorpion, the archer).
This method is a little tricky, because we can easily keep adding constellations and archetypes, nothing being affected. The archetypes we could work with are infinite, and the only limit would be how many can be added on the ecliptic. Because of this superficial understanding of the signs, we got theories about a 13th sign being added in the zodiac wheel. But the situation is a little bit different.