What Merchanht ships were declared as "protected" in WWII for the transferred of Prisoners of War, between warring nations
There is a book "Mercy Ships: The Untold Story of Prisoner-of-War Exchanges in World War II" which covers this topic.
The two principal ships used were Swedish: Drottningholm and the Gripsholm.
These ships were also used to exchange civilian internees. The total amounts of people exchanged amounted to a few 10s of thousands. In the case of soldiers, it was mostly sick or disabled soldiers that were exchanged.