A group or set of nine is an ennead - in ancient Egypt there was the group of 9 Egyptian deities known as the Ennead.
There are a number of cultures where nine is a significant number:
In Indian culture, nine appears in Indian astrology where there are nine universal substances or elements in the Hindu philosophy: earth, water, air, fire, ether, time, space, soul, and mind. According to yoga, the human body has nine doors - two eyes, two ears, the mouth, two nostrils, and the openings for defecation and procreation.
In Chinese culture, nine is considered a good number, as it sounds like the word "long-lasting". Nine is strongly associated with the Chinese dragon, a symbol of magic and power. There are nine forms of the dragon, and it is described in terms of nine attributes, has nine children, etc - all characteristics are multiples of 9.
In Greek mythology there were the nine Muses, and the Romans had nine gods.
9 gods aliens is a tag for the notion that ancient astronauts have visited earth and made contact with humans in antiquity and prehistoric times. One claim is that deities from most, if not all, religions are extraterrestrial in origin. The Erich Von Danker book Chariots of the Gods? was a best-seller on this topic. This idea is not taken seriously by academics.
How important is the number 9? The mystical numbers site had many references - some quirky and others intriguing. There's a moment of humour on the website with a quote from Ronald Reagan: The nine most terrifying words in the English language are, "Im from the government and I'm here to help."
The site also describes an interesting mathematical attribute: when multiplied 9 always reproduces itself 9x2 =18 1+8 = 9 9x3 = 27 2+7 = 9 9x4 = 36 3+6 = 9 9x5 = 45 4 + 5 = 9