The term "protagonist" is Classical Greek and literally means "first contestant". It is applied to the lead character in a Greek drama. Because the term means "first" there can be only one protagonist in a drama, or work of fiction, although there can be several important characters. It should be noted that not all literary works have a single protagonist; some works revolve around groups of equally important characters.
"A Descent into the Maelström" by Edgar Allan Poe has two main characters. The narrator is an unnamed tourist who listens to an account told by a Norwegian fisherman of the descent in the title. As the narrator really does nothing in the story other than listen to the tale and report it to us, he is not a protagonist.
The Norwegian fisherman who survives the descent is the protagonist because he is the crucial character around whom the actual events of the story revolve.