There are honestly too many factors to give an exact number. It all depends on the construction of the house and how the low power RF propagates in that building. You must also realize if you just span a distance of 180 feet with a switch every 40 feet then you have more of a chain then a mesh and any of those switches having a problem will kill communication. Do these clients just want to test the technology? If so, just test on 1 side with 3 switches and if they commit to it then do many more in between to optimize signal strength and the mesh.