Literary: What might Mormon have known of the five who went? He surely would have known that five went. That much would easily have made it into the record. What of this conversation? If it is true that “they said among themselves, as they went,” then Mormon should not have an accurate record of this conversation. There was no recorder playing as the men went, and even were they to have reconstructed this bit of conversation later, it is doubtful that it is more than a reconstruction.
It is just as likely that this is Mormon’s editorial insertion about what their conversation would have been. The import would have been available, and ancient writers had no problem in inventing dialogue to further their text. What we appear to have in this case is Mormon writing in such a way as to make his story more human by attributing dialogue. The literary effect of the dialogue is to bring us in to the action, and that is fully within Mormon’s editorial desire. Mormon wants us to be interested in this story, and he tells it for its dramatic qualities.
As history, we may assume that the overall facts are accurate, while allowing Mormon’s literary license in making the account more interested by invention some dialogue.
Cultural: Although there is no way to confirm it, it is possible that there was a cultural reason behind having five men sent. Why not six, why not two? In Mesoamerican culture there were five directions, one for each of the cardinal directions, and another for the center. Thus the sending of five could indicate a “complete” witness, or symbolically the “whole world” would be a witness if a symbolic five were sent.