Currently as can be gleamed from the data files, all ships from the little to the big, all get one move for each engine, so every ship can get 12 engines. It was noted earlier that the limit only refers to engines and not move, so any bonus move may go above 12...
The engine limits are merely x 2 of what they were in SE:IV. Essentially the whole movement system was multiplied by two, with the exception of fighters and drones... the ability description should also state engines rather than movement.
The SE:V Balance Mod returns to a more SE:III like propulsion model with larger ships requiring more engines - although it does make engines 20kT size so you need to only add half as many as in stock.