Up to 6 units is residential class. 7+ is MUR. The MUR rate is about 40% more than residential. There is an exception for more recent large builds which has a "New MUR" class that is at the same mill rate as residential. So it will depend on the building.
In my experience, if you want to add a unit and you are at 6 units already, you will need to get it to 10 units or it won’t be worth it.