It is far more efficient to build 1 ship with 5 cargo holds than it is to build 5 ships with 1 hold each. This is for a couple of reasons. The larger the ship the faster the construction speed is, so using 1 slipway will build a ship the size you need about 2.5 times faster per month. The larger ship is probably not 5 times as large as well so it will be fairly quick to build. 1 slipway will use less people to support it than 5 smaller slipways do, and will probably cost you more in minerals to build the 5 individual slipways. The second reason is that the overhead of crew quarters, any sensors, maintenance, cargo handling facility, ect is not that much more for the larger ship than for 1 smaller ship. Ie the larger ship will need 1 maintenance, 1 crew, and 2 cargo handling facilities. The smaller ship will need 1 of each, which is about 4 times more stuff. This goes up even more if you want any kind of sensors on your ships. I like putting a .5hs active sensor or thermal sensor. This lets you know when somebody else is in a system without increasing the price of the ship by much.
If you already have more than 1 slipway on the shipyard it will probably be faster to build a new shipyard and have it expand it's size as fast as possible. Somewhere between 35-40,000 tons should do for a minimalilst design freighter.
Brian