Dear @Zhicheng.Tang,
Regarding your first question, OpenFAST calculates sectional loads (forces, moments), but OpenFAST will not calculate stress directly. You could calculate stress from load via suitable post-processing, e.g., see the following forum topic for more information: fatigue calculation.
I’m not sure I understand your second question, but I’m not aware of any limitation to the number of potential-flow bodies in HydroDyn; that is, NBody can be larger than 9. Regardless, substructures where hydro-elastic effects are important are typically thin or small volume, where potential-flow is not necessary; rather, for such structures, the strip-theory formulation is likely a better approach.
Best regards,