If you are really struggling with draw calls, you can even attach dynamic elements together, like a flock of birds with each bird connected octopus-style to a root with the animation affecting the whole flock. It doesn’t have to be static.
That said, this is fine to think about, but, ultimately, your optimization should be driven by your real bottlenecks.
Like Darthviper107 said, objects that are joined are drawn together. This means that geometry that’s not in your view (because it’s occluded or literally not even in the camera’s view direction) cannot be culled if it’s bundled with geometry that is in your view. In other words, if you are nowhere near your draw call budget, and combine every object in the room together into a single object, then, when standing in the middle of the room with an FOV of 90 degrees, you will be rasterizing all geometry when the camera can only see a quarter of it (because the other 3/4s are in the 270 degrees you can’t see).
So you spent time… to lose performance… trying to optimize something that was never a bottleneck to begin with.
On the other hand, if submitting objects to the RHI is a significant portion of your frame, then yes, you can reduce that by submitting fewer draw calls. Every individual object in the scene contributes one draw call for every material it has. If you have Object A with three materials, Object B with two materials, and Object C with one material, then you have 3 + 2 + 1 = 6 draw calls. Combining A, B, and C will create Object X with six materials. Still 6 draw calls. If one of those materials is shared between A, B, and C, then you can make an Object X with just four materials, thus four draw calls… a 33% reduction in draw calls!
But… did you have to reduce the quality of the material? Is the material complicated, with multiple textures masked together, etc.? How long did you take to make it? If your draw call count was eating you alive, then this might be worth it. If not, then you might have actually traded off something that wasn’t a bottleneck by further-suffocating something that was.
Again, one draw call for every material of every individual object. Think about whether that’s a problem before wasting too much time / making a mess of your project, though. Your performance analysis tools (profiler, UE4’s stats commands, etc.) will point out your hotspots for you. You’ll eventually learn patterns and tricks based on past experiences.