A case is a collection of processes. Generally, the make up of the case itself determines the relationship structure. As the case is reviewed in DCO sessions, the relationships and structure is determined. A few best practices:
A case shoudl not be larger than 7 plus or minus 2 Stages.
A process should have no more than 15 shapes.
However, it is really the needs of the business and than the functionslity desired that help to shape the case structure.