Albert Barnes on Luke 16:27-28

Five brethren. The number five is mentioned merely to preserve the appearance of verisimilitude in the story. It is not to be spiritualized, nor are we to suppose that it has any hidden or inscrutable meaning.

May testify unto them. May bear witness to them, or may inform them of what is my situation, and the dreadful consequences of the life that I have led. It is remarkable that he did not ask to go himself. He knew that he could not be released, even for so short a time. His condition was fixed. Yet he had no wish that his friends should suffer, and he supposed that if one went from the dead they would hear him.