Prime mover size and number are typically dictated by tradeoffs considering manning, redundancy, operations, economics, plant architecture, as well as fleet issues (maintenance) and tradition.
An integrated plant (e.g., propulsion & ship service, etc) for a ship employing electric propulsion will be different for a split plant arrangement, or for a plant employing electric power take-offs from a mechanical shaft, and all that will be different for a combatant versus a cruise v. commercial vessel. Redundancy and continuity of mission services will, of course, have more weight in a combatant than in others. However, all that being said, traditions and subjective considerations sometimes factor in.
With respect to space, weight efficiency, having one larger plant most often requires less space/weight than having multiple smaller plants for a given shaft horsepower especially when considering the support services needed. For instance, consider the support services required for a diesel engine in the way of lube oil, cooling water, air, exhaust, monitoring, and control, etc. Space and weight are more important in a ship than in a landlocked structure and thus given more importance in optimizing a plant.
