Thank you for the recommendation to make use of Random Stream 
I switched to a C++ implementation however I’m having an issue when I go in Play mode. My Instanced Static Mesh just disappear 
Any idea what might be the problem here ?
Oh and do you do for the lighting ? I tried to build it and I get a warning: “Warning LevelGeneration The total lightmap size for this InstancedStaticMeshComponent is large, consider reducing the component’s lightmap resolution or number of mesh instances in this component”