When someone asks for some lemons, it's no big deal. However, when someone asks for, say, three lemons, it makes us wonder what those lemons might be for and why they would need such a specific number of them.
In several places, Coleridge provides a very specific number for things. By now, we know he's such a sly dog that he'd never give us such a specific description without meaning something by it. Though a couple are more folklore-related, most of the numbers are religious symbols of some sort. That's no surprise, considering that symbolic numbers pop up quite a bit in just about every religious tradition in history.
Line 10: The numbers here tell us that it's midnight. The poem opens right at the start of the "witching hour," that hour of the night where the veil between the world of the living and the world of the spirits is thinnest and when witches and other spooky folks have the most power. This is yet another neon warning sign that something bad is about to happen.
Line 12: We already know that it's midnight, and that the dog howls once each quarter-hour plus twelve times for the hour—just like a clock tower. Seems simple enough, right? Well, maybe not. Just as Coleridge gives us a little math lesson to remind us that twelve plus four is sixteen (thanks for that, Sammy), he drops the bomb that there's a ghost hanging out with the dog. Reading forward a little, we understand that this ghost is Christabel's mother. Placing the number and the appearance of Christabel's mother so close together (especially when her sudden appearance seems like a total non-sequitur) makes us feel like the speaker is trying to tell us something. In the Bible, the number sixteen shows up as a symbol of God's never-ending love. Here, it may symbolize a mother's never-ending love and protection for her child, even after that mother is dead.
Line 49: The number one is singular, of course, but it is also contained in everything else (we can't have two sandwiches without having one to start with). Here the one leaf represents the presence of two opposing forces within one thing—spring and winter, life and death, good and evil. It's telling us that within us of all there exists the potential for both good and evil. Hey, it looks like one isn't so lonely after all.
Line 81: In Christianity, the number five represents God's grace. In some folklore, it's only a pack of five men on white horses who can hope to catch a vampire. Again, this gives us a pretty big hint about Geraldine's true nature.
Line 201: Bells are a traditional part of a wedding day, so talking about bells being rung on that day is no big deal. However, Christabel specifically says that the bells will be rung twelve times on her wedding day, which could be considered in a couple different ways. First, as a Christian symbol, the number twelve is supposed to be a sacred number. It symbolizes God's power and authority. Ringing the bells twelve times on a wedding day would be a blessing for the couple, letting them know that their bond has been made with the authority of God. That would probably make Christabel's mom happy, even in the afterlife. What probably won't make Christabel's mom happy is that it's possible that Coleridge intended for us to understand that Christabel is already married—to Geraldine. Remember the beginning of the poem when the narrator tells us the bells rang twelve times to mark midnight? Christabel's mother was there to hear them too if the dog really is seeing her ghost. To add to this weirdness, Christabel even carries her bride over the threshold of the castle in lines 131-132. It's not a literal marriage, of course, but it does appear that, literal or not, the couple consummates that marriage that night as well. Christabel's mother doesn't show up again as a ghost after she's shooed away by Geraldine in the room that contained their wedding bed. It's layers like this that make this poem so interesting, and very likely explains why Coleridge worked so hard on it yet never finished it. This level of complexity is hard work.
Line 305: One single hour is all Geraldine needed to completely corrupt poor Christabel. Specifically, that one hour was the witching hour, so that probably helped make the job easier.
Line 341: This one is a bit tough. We know it's important, since the Baron has specified such a specific number of beads (or prayers) that the sexton must pray each morning with the bells. Still, why 45? The number has no real significance in Christianity, paganism, or anything else, really. Perhaps Christabel's mother was 45 when she gave birth (which could help explain why she died, since older mothers are at higher risk for complications during pregnancy and childbirth even today). All the same, it seems highly unusual for a woman that age to be giving birth at that time. Perhaps its significance is in the fact that the number isn't significant at all. It may be a symbol of how Sir Leoline is just a little bit crazy due to the grief of losing his wife. Only a mad man would be so very specific with something so wildly arbitrary.