The Rigveda does not know how many Adityas there are. One hymn names six. Another says that Aditi has seven sons, and a further passage has her bring seven forward and cast the eighth away, so that Martanda, the dead egg, is then brought back as the ancestor of mortals or identified with the sun. The variation is not a textual problem to be corrected. It belongs to a body of poetry that was not attempting a fixed pantheon in the first place.
The number twelve is later. It appears in the Brahmanas and is standard by the time of the epics and the Puranas, where the twelve Adityas are matched to the twelve months so that a different Aditya presides over each, and the class becomes explicitly solar. Later lists include Vishnu among them, usually last and often as Vamana, which is how a god who is marginal in the Rigveda acquires a Vedic pedigree, and they include Surya, who in the Rigveda is a separate deity and not a member of the class at all.
Two things happened at once in that development. A group defined by sovereignty and law became a group defined by the sun and the calendar, and a fluid count became a fixed one. The atlas holds the class as a class for exactly that reason: the membership is a moving thing across the literature, and pinning it to any single list would misrepresent the sources it is drawn from.