Bram Stoker clearly lived by the ethos of "the more the merrier." There are definitely more narrators in this bad boy than in your average novel.
The novel is composed of a series of journal entries, letters, newspaper articles, and memos. Bram Stoker explains the rationale for this structure in a brief note before Chapter 1 (See "What's Up With The Epigraph?" for more on that).
In order to make the story seem realistic, Stoker presents the novel as a series of supposedly "real" documents—the reader is given just the facts of the case, written out by the people who experienced the events directly. This narrative technique puts the reader in the position of a judge or jury (or both): We hear the evidence of a variety of different eyewitnesses, and we're supposed to interpret the it as best we can... given that it's totally otherworldly and terrifying.
We're not given a central third-person omniscient narrator who can tell us what to think about the events. Another effect of this technique is that we hear about the same events from multiple perspectives—we have access to multiple points of view, so there isn't just one character we sympathize with. We pretty much like 'em all.