Sorry I wasn't clear in my previous reply. Yes, it will definitely need 1 flow per source structure, I think even Harish's suggestion requires 2 flow per source structure. I personally would not go for multiple operations in one service interface as it can get messy.
We've done this two step approach in one of my previous project as a rationalization on the architecture. For each new B2B partner integration, there was always going to be an interface that maps to an ORDERS IDoc, and redoing the mapping for each was just repetitive. After changing to this approach, adding a new partner was much simpler - we had to map much fewer fields from the partner specific structure to the intermediate structure compared to if it was the ORDERS IDoc.
The challenge though is to come up with a multi-purpose intermediate structure and developing the 2nd common mapping in a generic manner.