There are many narrators (inner and outer) in the novel. The first narrator is Walton. He writes letters to Saville. Walton, then, is the outside narrator. You may say that he narrates the entire novel as all of narratives are filtered through him. He is the "survivor" at the end who carries all the stories back to civilization.
Then, there's Victor, who tells his story to Walton. He is the inner narrator, as most of the story is told by him, but again--it is relayed back to us through Walton.
Below is a table that shows the narrator / audience dynamics:
Narrators 1st Aud 2nd Aud 3rd Aud 4th Aud 5th Aud
Walton: Saville Reader
Frankenstein: Walton Saville Reader
Monster: Frankenstein Walton Saville Reader
De Laceys: Monster Frankenstein Walton Saville Reader
He told his story to Robert Walton after Walton rescued him.