In my story, 12 year old Ruth has visions from another place and time which lead her to gather a group of kids for a quest. She is told there will be 18 kids, but she can only find 17 with the specific requirement. They travel together and, upon arriving, discover a stowaway. This boy knows he belongs with the group but he doesn’t tell anyone until well into the book (the reader also doesn’t know). Everyone else thinks he just came along to annoy them. The 18th child is an important character and his journey is pivotal to the story.
Why 18? Because it’s a Jewish-themed book and 18 is an important number in Judaism (it is related to the word for life and is often used for luck).
There is no specific task that requires 18 people and there is no need to highlight the number too much. It's there because it felt right to do so and, at this point, the characters are set. The number comes up a couple of times early in the story and, so far, not since. I'm looking for subtle ways to reference it.
In what ways can I evoke this symbolism and incorporate it into the story?