For stations and facilities you'll want to use the maximum allowable amount of builders, if you want it built as soon as possible. Which is 25 for stations.
You can use between 1 and 25 as Ten said - the only difference is the time it takes. There is no change in cost or anything else.
As for the DCs, not all the paramilitary factions get the DC as part of their faction setup. Whether they can get it for free depends on how friendly they are with factions that do have access to them.