Applying a different material to a polygon will add draw calls. And it’s really unnecessary because it’s a background object and doesn’t need to be complex.
If you really want to, you can do something like that in your 3D software first by adjusting the UV’s which will allow you to use just one material.
In any case, it’s really simple to texture a building with windows and make it more randomized if you need to without doing anything crazy, it’s just UV mapping.