You only need one smoothing group for source engine colission models. Just make sure your elements are not welded together or something. There is however a limitation of how many elements a colission model can have.
@DEElekgolo Thank you. But sadly when making a Collision model for Source Engine games to have concave collisions rather then the engine generating a custom model that convex on every outer vertice of the reference model. You have to have each model(element) on a separate smoothing group or else elements with the same…