Basically you need to implement chunking.
Only the active block you are in is made of individual drawcalls, stuff out of that immidiate range is x2 - and progressively bigger power of 2s the further you get probably.
When you build the meshes you know where the tris are supposed to be if you use cubes (minecraft style?) So the rendering cost is miniscule as all you need to store is a boolean value of “block existence”.
You then have something like a 9x9x9 set of blocks, and all you need to make the mesh build for the chunk is the 0 or 1 value of each block…
Still, you can have billion instances before stuff geinds to a halt using HISMC…
One last thing.
@eray_ozr is wrong.
The instances are directly accessible by the engine in blueprint regardless of what they are, how many, etc. Without any issue or slowdown.
The engine will geind if you populate an HISMC and then try to edit the instances in engine. That is because the editor wants to read all the instance information at once.
Otherwise, all it takes is a line trace to get the index.
DynoFoliage: UE4 Interactive Foliage works for foliage approximately that way (but accessing foliage is a bit more complex).