The optimum is based on land value, clearly, because in places where the land is effectively free or incredibly low cost, buildings don't seem to naturally get above 2 or 3 stories.
So somewhere between 3 and 12 stories is a local optimum, and it's probably about where you need additional "machine floors".
Where land is free 5 floors is the optimum just because nobody will accept more than 3 without expensive elevators (it may not even be legal if you would accept it!). Most people are willing to pay a little bit more for a larger foundation just for less floors.
For car oriented locations 1 floor is optimal because the type of building that can handle more than one level of cars is so expensive you want to just build a large parking lot. (In theory the humans could be in taller buildings, but the space used for humans becomes a rounding error, and so you may as well avoid the expensive elevator)
So somewhere between 3 and 12 stories is a local optimum, and it's probably about where you need additional "machine floors".