The answer to this question depends on the size and layout of the building. Generally speaking, an Air Handling Unit (AHU) should be installed on each floor of a building if the floor is large enough to require its own air conditioning system. If the building is smaller, then a single AHU may be sufficient to provide air conditioning to all floors. Additionally, the AHU should be sized appropriately for the floor it is servicing, as an undersized AHU may not be able to provide adequate cooling or heating. It is also important to consider the type of air conditioning system being used, as some systems may require multiple AHUs to provide adequate cooling or heating.