Solo,
One of each of the bells, wind mill, and air conditioner will stack or have a cumulative effect. Multiple of any of the items does not stack or have cumulative effect.

NMF,
The quantity of manufactures with the mill feature vary a bit. Max is 50. Furniture and anything that is put onto floor after crafting (example: pet-top) is limited to 1. You can combine the features of the Air Conditioner and Mill, but you have the same restrictions as the Air Conditioner.

Spoiler:

Level 156 fire
T4
90% of the laws are made because of 10% of the people.