The longer the answer the better, so please elaborate
Normally fifteen different narrators would be overwhelming. The text would be confusing and scattered. As I lay dying, however, is was written by William Faulkner. In his expert hands we get to soak up an array of narrative styles yet keep an intimate unity. Check out this from GradeSaver,
The structure of As I Lay Dying is powerful and innovative. Fifteen narrators alternate, delivering interior monologues with varying degrees of coherence and emotional intensity. The language is intense and highly subjective, with a recognizable change in language depending on the narrator. Each section falls somewhere in the range from confessional to stream-of-consciousness. The novel is a series of interior monologues, and through these fragmented passages we piece together the story of Addie Bundren's death and the transport of her body to Jefferson.
The narrative appears fragmentary, but the story demonstrates admirable unity: it is limited to the span of a few days, and the different sub-plots are logically and skillfully interwoven. Faulkner's innovation is in how we see this unified set of events: we are forced to look at the story from a number of different perspectives, each of which is highly subjective. In The Sound and the Fury, Faulkner made use of some elements of this technique. However, As I Lay Dying presents us with a far greater range of voices. Additionally, The Sound and the Fury provides a clearer distinction between unreliable and reliable narrators. Part Three of The Sound and the Fury is narrated by a man who is unmistakably evil, and Part Four helps clarify the novel through its use of a more objective third-person narrator. The voices in As I Lay Dying are more numerous and more ambiguous.
Many people don't even realize how often the point of view changes in this novel, and when they do....... they're maybe two to three chpaters into the reading.
The disadvantage of 15 different narrators would be what I've already cited; the reader gets confused.
The advantages are numerous. Different points of view allow the story to progress quickly, they keep it fresh, never boring. We see the story from every angle and meet numerous personalities along the way. Once you've figured everyone out, it becomes a fascinating read.
Use of different narrators also allows the reader an opportunity to relate the text to their own life. If you don't relate to one character, you may relate to the next. This gives the reader a stake in the story, something many novels don't do. If your reader can relate, empathize, or become emotionall invested in one or more of your characters...... you've been successful. This novel accomplishes that.
