This number will vary a lot among data centers. In our university environment, we have sites between 2 and 20 kW/rack, depending on the location and the type of equipment. My advice is to use your own best information to install reasonable space, power and cooling capacities at the start and have a solid plan for increasing capacity in case you run out. Remember, unfinished spaces and pathways (conduits, ducts) are cheap and easy to install during new construction and easy to grow into once the data center is operational. Remodeling a live data center is expensive and disruptive.
Certainly there are many experienced designers available to help you with this and other capacity planning issues -- see
http://www.datacentertalk.com/catego...sign-services/.
Ken