Well, the part that's up to a developer is how AI and entity simulation is managed, and the models. As for the rendering of a large number of objects, that will have to be supported by us. The limits mostly depend on quality of LOD management for objects, provided the models come with proper LOD representations.
We are quite focusing on it, the size of the world mandates that it is tuned well. Also, doing thing in a way that plays well with graphics hardware helps a lot; otherwise OT would not be here at all