Nine is the highest digit, hence symbolizes comprehensiveness, culminations; also, the reason it is used in the Temple's form is because 9 has exact numerical value of ' Baha' (in the numerology connected with the Arabic alphabet) and ' Baha' is the name of the Revealer of our Faith, Bahá'u'lláh.
The 9-pointed star is not a part of the teachings of our Faith, but only used as an emblem representing '9'. In telling people of the 9 religions of the world, that is 'existing religions, we should not give this as the reason the Temple has nine sides. This may have been an idea of the architect, and a very pleasing idea, which can be mentioned in passing, but the Temple has 9 sides because of the association of 9 with perfection, unity and ' Baha'." (Shoghi Effendi, Directives from the Guardian, p. 52)