An interesting question that goes to the root of their meaning; and to say that the zodiac signs are twelve like the months of the year only displaces the problem
Some may think that their number corresponds to that of the actual constellations. The fact is that the constellations are a fictitious construction created at the time for mnemonic reasons, that is, to give the possibility of setting reference points in the observation of planetary motions, etc. Therefore they cannot explain the number of the zodiac signs.
The lunisolar year, still used in many cultures today, measures the number of lunations in a solar year. The lunar or synodic month (the time interval from new moon to new moon) is approximately 29 days. There are, therefore, twelve lunations in a solar year with a surplus of about eleven days; this means that we have to insert the so-called embolismic or intercalary days or months to reintroduce the synchronism between lunar months and seasonal periods. Although, in this case, there is a precise astronomical reference to the number in question, the phase shift between the solar and lunar years is a source of a lack of symbolic identity because the Zodiac is based on the seasonal analogy.
Let us now consider the question from the point of view of numerology symbolism. As the solar system’s central star, the Sun represents unity, the creative potential. The Earth embraces this potential by making itself receptive to it and originating nature, the earthly response to the solar principle. In this genesis, we see represented, on the plane of manifestation, the first three numbers: the One, which in the act of knowing itself is reflected in the Two, the Binary, which reveals the Three, the Ternary. The latter is the fruit of the reinstated original unity, the life reflecting the One in the manifold. The Four or Quaternary fixes the Binary (2 + 2), and the resulting stasis brings the generative principle to density.
In zodiacal symbolism, the Ternary division refers to the three Crosses of the Elements (Fire, Earth, Air, Water, to be understood not strictly as physical elements but as modalities of matter). Each Cross brings together the strength of the four Elements. The Quaternary division (the four triangles of the Elements) inscribes in the zodiacal circle the transitory modalities of the Elements themselves: their original initial state, the intermediate or fixation one, and the final or transformation one. Each Zodiac sign is associated with an Element, and each Element in its three phases are assigned three Zodiac signs. The product of the Elements with the triangles of its evolutionary states is 12, representing in the Zodiac the tripartition of the four seasonal phases; this is how nature lives and thrives in an ongoing transformation process.