I´m not talking about static meshes itself, but about static mesh foliage…
Read by yourself:
Static Meshes placed with Foliage Edit Mode are automatically grouped together into batches that are rendered using hardware instancing where many instances can be rendered with only a single draw call, while Actor Foliage comes at the same rendering cost as placing normal Actors into a scene.
Can be found there: Foliage Tool | Unreal Engine Documentation
And more info about that: Instanced Static Mesh | Unreal Engine Documentation