Knowing that if you deplete the battery to near zero that you cause the life of the battery to be reduced. Does it make sense to double the amount of batteries to make sure you only deplete your batteries halfway?
Actually anything below 50% is bad.
I think the best thing to do is figure out how many amp hours you need and size the batteries accordingly.
IE what do you want to power? Lighting, 12v diesel heater etc. do you plan to invert for TV, microwave?
AGM chart for battery capacity/state of charge.