How do you tell your reader that a number is a numerologically important number? Some people use numerology in their books and attach secret meanings to some numbers, but there are a lot of times where you would use numbers without wanting to attach a secret meaning to it. How do you distinguish normal numbers from those with special numerological relevance in your writing? Can you give a few examples on how to do this?
I'll preface this answer by noting that there's no way to make sure all your readers will pick up on something, no matter how obvious you try and make it. So don't worry about making it too obvious, because readers who already did pick up on it may feel as though you're insulting their intelligence.
Having said that, I think you can rely on simple pattern recognition - if the same number keeps appearing in your story, then people are going to pick up on it. The film The Number 23 revolves entirely around an in-universe example of this, where the protagonist keeps noticing the number 23 and it clues him in to a wider mystery.
I've used this myself in one of my own story universes: it has its own numerology, tied to its creation myth, wherein six is a "good" number and eight is an "evil" one. As a result, I try to find subtle ways to incorporate those numbers into the stories in that universe, in good/bad contexts respectively. The aim is that, by doing that, people will pick up on the fact that those numbers keep appearing in those contexts, and in turn pick up on the numerology.
Explicitly inform the audience of the numerology.
I'm sort of reminded of the web serial Unsong, which revolves around Jewish mysticism being real to the point where it can be industrialized, and which repeatedly states throughout the text that nothing is ever a coincidence as a result. So, in such a setting, if you want to add in some numerology, you could just go and say something like "The number of Mr. so-and-so's apartment was 738, which is also the value of the word "devil" in Jewish gematria. This is not a coincidence, as nothing is ever a coincidence, and Mr. so-and-so truly was a devil."
Make it explicit, mention it in-universe, and bring it up whenever it's plot relevant, and maybe a few times when it isn't.
Sometimes you have numbers that mean nothing, and sometimes you have numbers that mean something. How can you distinguish between them without shoehorning it in or shoving it in the readers face, spelling it out, making it corny?
Have a character or the narrator notice that number more than once, or briefly fixate on it.
From "The Colour of Magic" by Terry Pratchett:
"Don’t say it!” he hissed. “Don’t say it and we might get out!"
“Get out? How did you get in? Don’t you know-”
“Don’t say it!”
Twoflower backed away from this madman.
“Don’t say it!”
“Don’t say what?”
“Number?” said Twoflower. “Hey, Rincewind-”
“Yes, number! Between seven and nine. Four plus four.”
Rincewind’s hands clapped over the man’s mouth. “Say it and we’re doomed. Just don’t think about, right. Trust me!”
“I don’t understand,” wailed Twoflower.
Rincewind relaxed slightly; which was to say that he still made a violin string look like a bowl of jelly.
“Come on,” he said. “Let’s try and get out. And I’ll try and tell you.”
This is after prior discussions about how wizards shouldn't have anything to do with the number 8. This is satire, of course, but it gets the point across about it being okay to be obvious.
I have three suggestions that I think can be taken as points on a continuum, from “spell it out in very plain terms” to “don’t tell them anything whatsoever”.
Tell them outright
Maybe your characters discuss numerology and point out the significance of the numbers. Or it could be internal monologue, or narration, or whatever. But the text explicitly calls out which numbers are meaningful, and what their meanings are.
Give them the tools
You could incorporate enough information into your writing to act as a primer on whatever sort of numerology you’re into, without specifically applying it to your significant numbers. Maybe a character analyses a number that isn’t plot-relevant, demonstrating techniques a canny reader could use on the numbers that do have story significance.
Assuming your numerological system is a real-world one, you might play it coy, letting (hoping!) your readers pick up on the use of numerologically significant numbers without you ever doing anything to point them out.
As I say, these are points on a continuum, and you can interpolate other possibilities from them. I daresay this kind of spectrum could be used for all kinds of information in stories (and I daresay someone’s come up with it before and explained it better than I could).
Rather than a writing solution, I would suggest a typographic solution. If a number is intended to have special meaning, to disambiguate it from other numbers that happen to appear due to plain description and no secret/special meaning, have it emphasized via bold or being in another color.
The two gentleman went to the counter and ordered thirteen pretzels.
"Hey, what was the number for the pizza place again? Was it 123-777-4567?".
Beyond that, as others have suggested, somewhere in the text of the story there should be some explanation into which numbers are important and why, but this need not occur before any such bolded or emphasized numbers occur in the text.