Large-scale water electrolysis systems are built from a multitude of stack units. These stack units have to be controlled during operation. The question arises how process control of the system can be designed in that way, that scalable, extendable but also optimal operation of the stack units becomes possible. To achieve this goal, existing process control strategies from literature are analyzed. The requirements resulting from the inherent numbering up of stack units are described and an approach for an adaptable optimized process control strategy considering the system properties is proposed. The proposed process control strategy is implemented in a modeled electrolysis system and evaluated regarding the stated criteria. The results could show, that the inherent modularization of electrolysis systems can be used in advantage for a scalable operation.