Amazon has close to half a million servers, according to research done by Huan Liu of Accenture.
This is impressive, though not as big as Google which according to my estimates of almost two years ago has around two and a half million servers (see Google server count).
Of course, both these numbers are guesses and are based on quite a few assumptions. Nevertheless, they should be in the right ballpark.
Huan Lui figured this out by checking for patterns in assigned network addresses. My approach was Google’s financial statement.
Interestingly, the US-East facility of Amazon in Virginia has more servers than the six other regions combined.