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.
You can build 100 ships at one time, if you can fit all the materials for the 100 ships in the shipyard. Also keep in mind if it takes 10 days to build one ship it will take 10 (days) x 100 (ships) to finish the entire order.
Though you can start production, and then queue up the other 99 ships so they finish one at a time. If you go that route though you can queue up the 99 ships without needing the materials at that exact time. When each ship finishes the shipyard will check to see if the materials are there to start the next one, and if there are then it will auto start building the ship. So you can bring in materials as you need them.
I believe, and I say this having only done it once before, that you have to have the worker npc's and the shipyard/factory assigned to a faction of the correct type(ie. ship production faction for ships)
Oh, and you need to use the right amount of npc's, they can be found on the production rules page.
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