The reason number nine is considered by the Bahá'ís as being sacred

The number nine, which in itself is the number of perfection, is considered by the Bahá'ís as sacred because it is symbolic of the perfection of the Bahá'í Revelation, which constitutes the ninth in the line of existing religions, the latest and fullest Revelation which mankind has ever known. The eighth is the Religion of the Báb, and the remaining seven are: Hinduism, Buddhism, Zoroastrianism, Judaism, Christianity, Islam and the religion of the Sabaeans. These religions are not the only true religions that have appeared in the world, but are the only ones which are still existing. There have always been divine prophets and messengers, to many of whom the Qur'án refers. But the only ones existing are those mentioned above. (From a letter written on behalf of Shoghi Effendi to an individual believer, dated 28 July 1936; The Compilation of Compilations, vol. I, Extracts from the Baha’i Writings on Buddha, Krishna, Zoroaster, and related subjects)