A total of eight
plaques of this type were flown on the seven manned
lunar landing attempts. Six of them remain on the moon to this day.
The discrepancy between the number of plaques, the number of flights, and the number remaining on the moon is caused by Apollo 13. Because of the last minute swap of Jack Swigert for Ken Mattingly, the plaque mounted on the Lunar Module now had the incorrect crew names and signatures. Because the LEM was already stowed inside the Saturn V booster, the plaque attached to to it's landing leg could not be changed. A new plaque was manufactured and stowed inside the Command Module. Once on the lunar surface the crew was supposed to attach the new plaque over the top of the incorrect one.
Because of the explosion in the Service Module, the planned lunar landing was aborted. The original plaque, still attached to the Lunar Module, was destroyed when then the LEM was jettisoned and allowed to re-enter the Earth's atmosphere. The replacement plaque was retained by mission commander Jim Lovell as a souvenir of the mission.