Items, droids, weapons run in batches, because otherwise it makes no sense to be using so many RMs for one item (or conversely, using fractional RM amounts). Ships and vehicles etc however are large enough that such steps do not need ot be taken, so each DC use is for 1 ship.
However, as Myrhrai said, you can setup factories/shipyards to build more than one ship at a time - however you would need more DC usages, plus adding more ships to build at one does increase the time for each additional use. Only really use for such 'batch' production is either if you feel that the DC usages will be yanked, or its for fighters and you cannot be bothered popping back every couple of days to start production up again etc.
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