BC means Boundary Condition, sorry I wasn't clear on that abbreviation.
Bonded contact can be used to represent thermal resistance at the surface without needing to explicitly make a body with a different material to represent that surface layer. For example, a layer of oxide on the surface of a metal that has a different thermal conductance than the parent metal. You can represent that layer in the Bonded Contact definition itself. If you don't need to do that then you are better off without any contact elements.
If you look top down on a wireframe of the assembly, are there any planes of symmetry in the geometry, materials, loads and BCs? If so, you can cut down the model size by using a Symmetry BC. If all those items are not symmetric then you need a full model.
Please describe the BCs on all faces of the model. If you don't define a BC on the face of the model, it is automatically considered to be insulated (zero heat flux).