As was stated, you have to know the total load, that all the sockets added together will be expected to handle. Then your single wired circuit from the battery, will have to have the proper AWG wire and circuit protection for that amperage draw.

The sockets can be wired in parallel, with the hot wire jumpered from one to another. You will have to case ground each receptacle.

If you are going to use the automotive type of 12V receptacles, ( the cigarette lighter type), the grounding part of the case, the external circular tube, has to be addressed. The fixture has a hot wire lead that goes to a centralised, internal spring loaded terminal. Some fixtures have an additional ground lead that is affixed to the case.

Many times an adequate ground is effected, simply by attaching the fixture to a the vehicle's metallic structure. If this is not possible, an external ground wire will be necessary. There are two ways of addressing this. One is to run a lead to the nearest point, where a stud can be insterted into the metallic frame, and the ground wires are individually terminated. The second is to use a common grounding point such as a terminal strip, and run a two wire system, with one being the hot leads, the other the ground wires.

There are advantages to each method, the best will be dictated by your individual circumstances.