That's assuming every watt that goes into the data center gets to the IT load. Though it says in the documents for the facility that they're going to make it energy efficient, power still needs to be used for air conditioning, redundancy, facility lighting, security, etc. Assuming a PUE of 1.5 (PUE is total facility power divided by IT load), which is very efficient, you're talking about 85,000 servers.
But even that assumes all the IT load will be for servers. Certainly there will be power going to servers, network switches, etc., so the total would be lower than that. And if the NSA is using any larger servers (which considering its history, it most likely is), the number could be substantially lower than that. The average power consumption for a TOP10 supercomputer in 2008, for example, is about 1.3 megawatts, which in itself equals 2,600 500-watt servers.