Concerning the number nine; the Bahá’ís reverence this for two reasons, first because it is considered by those who are interested in numbers as a sign of perfection. The second consideration which is the more important one is that it is the numerical value for the word “Bahá”.
(B = 2, h = 5, a = 1, and there is an accent at the end of the word which is also = 1; the ‘a’ after the ‘B’ is not written in Persian so it does not count.) In the Semitic languages—both Arabic and Hebrew—every letter of the alphabet had a numerical value, so instead of using figures to denote numbers they used letters and compounds of letters. Thus every word had both a literal meaning and also a numerical value. This practice is no more in use but during the time of Bahá’u’lláh and the Báb it was quite in vogue among the educated classes, and we find it very much used in the Bayan. As the word Bahá also stood for the number nine it could be used interchangeably with it. Besides these two significances the number nine has no other meaning. It is however enough to make the Bahá’ís use it when an arbitrary number is to be chosen.
(Shoghi Effendi, Lights of Guidance, p. 414)

First, regarding the significance of the number nine; its importance as a symbol used so often in various connections by the believers lies in three facts: first, it symbolizes the nine great world religions of which we have any definite historical knowledge, including the Bábí and Bahá’í Revelations; second, it represents the number of perfection, being the highest single number; third, it is the numerical value of the word ‘Bahá’.
(Shoghi Effendi, Lights of Guidance, p. 414)

Nine, as the highest single-digit number, symbolizes completeness. The Bahá’í Faith regards humanity as an organic entity which has developed through its embryonic state to infancy, then to adolescence and is now coming of age, which is the state of fulfilment; so likewise the number nine reflects a sense of fulfilment or culmination and perfection. Bahá’í Houses of Worship, for example, have nine sides, and one commonly used symbol of the Bahá’í Faith is a nine-pointed star. Nine is the minimum number of members in a Spiritual Assembly or House of Justice, the administrative bodies elected by Bahá’ís around the world.
(Universal House of Justice, The Nine-Pointed Star: History and Symbolism, 24 January 1999)

Nine is the highest digit, hence symbolizes comprehensiveness, culmination; also, the reason it is used in the Temple’s form is because 9 has the exact numerical value of Bahá (in the numerology connected with the Arabic alphabet) and Bahá, is the name of the Revealer of our Faith, Bahá’u’lláh.
(Shoghi Effendi, Lights of Guidance, p. 415)

The nine religions to which you have referred include both the Bábí and the Bahá’í Dispensations, Bahá’u’lláh being the ninth Prophet in the series. The other Prophets included are Zoroaster, Krishna, Moses, the Christ, Muhammad, Buddha, the Prophet of the Sabaeans Whose name is unrecorded, the Báb and Bahá’u’lláh.... Buddha appeared in the Adamic cycle.
(Shoghi Effendi, The Compilation of Compilations vol. I, p. 20)

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.
(Shoghi Effendi, The Compilation of Compilations vol. I, p. 19-20)