Each Plenary is for one person. When you get the Plenary, it’s for you, but you can ask God to please give it to a soul in Purgatory. So if you want to get a plenary for 50 deceased people, you must earn 50 of them and ask God to assign them one to a person.
That’s very nice you want to get them for the deceased LV victims. You can only get one plenary per day maximum, and each one requires a Holy Communion of its own, so this will take a while, but you can also ask God to assign any partial indulgences you might earn each day to them too.
