My understanding is that drawing all instances takes 1 draw call, so all instances are drawn at the same time. However, I believe the engine is culling anything that is not being rendered (occlusion culling), so no worries about having to do anything with the stuff off camera.
You might want to check out Hierarchical Instance Static Mesh component - this is a better version of the ISM component. Now you can use LODs between instances and define distance culling for even more performance!
Hope this helps!