A researcher has taken the time to calculate how many servers are being used for EC2. Mturk runs at the Virginia location.

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And now, a researcher with Accenture thinks he has the answer: 445,000. That’s the number that Huan Liu came up with when he did a bit of internet sleuthing. “It’s a fairly big site; it’s pretty impressive,” he says of the entire EC2 operation.
EC2 is Amazon’s pay-as-you-go computing service. It’s become a popular way to spin up computing power for a corporate skunkworks project or a startup, but it’s also the back-end for serious online sites, including Netflix and Dropbox.
Liu’s analysis found that Amazon’s main cluster of data centers, located in northern Virginia, is truly massive: he guesses that Virginia is home to about 322,000 servers. But he also found that Amazon has a relatively small footprint in other parts of the world. For example, he guesses that there are only 1,600 EC2 servers in Sao Paulo, Brazil. It’s “hard to compete with Amazon on scale in the US, but in other regions, the entry barrier is lower. For example, Sao Paulo has only 25 racks of servers,” Liu wrote in a blog post discussing his findings.