Hello guys;
When I was watching the video i was a little bit annoying about building all these walls it will consume memory and time. So my question is : is it possible to just place one wall and just scale it ??
thanks in advance 
Sure you can. But think about what you are going to do next with your level.
If you are planning on using the basic materials, i recommend you not to scale too much the meshes.
The textures will not look good in that way, unless you really get into the Material Editor.