factors that determine rack capacity
 

what determines the number of servers that one can store in a rack other than the physical space?
if it is power limitation, why cant we just connect another PDU and add more servers?

As power density increases, so does heat, so your other limiting factor is adequately cooling the equipment in your racks. This means removing the hot exhaust air from the servers and supplying adequate intake air. A good rule of thumb is that airflow needs to be around 150 cfm/kW. In very dense racks, careful cable management is necessary to insure that wiring does not become an impediment to airflow.


Ken

Ken has given a nice example of the thumb rule about airflow.Another factors which can also be considered:
1 location of rack in datacenter rows: is the rack in Hot side or cold side. airflow will accordingly affect.

2 Weight of eqipments.

3 PDU ofcouse

4 Bandwidth requirement etc

and list goes on...