The functional part I agree with you “moved on” to more detailed indoor tagging. I’m talking more about the part, that there should be anything different between one building:part and another. Like the height, the actual roof type, … building:part was never meant to use it to create regular shapes for easier 3D-modeling.
Again, I don’t want any renderer to support that proposal. Just saying, this is all what currently is defined and either “we” should come up with something better/more feasible or we should follow, what is documented.
If this in anyhow helpful, I can think about a logic.
Why do you think the number is needed? There is an existing tag to describe the angle of the roof. With roof:angle, overall length (from the geometry) and roof:height you can calculate the quantity.