I have always wondered about this, and speculated about the number. A few times, I have even tried to calculate the number of AdSense publishers.
Of all my attempts to guess the number of publishers, I never guessed that high. How about you? Are you surprised?
So, the next time you wonder if you have much competition, now you know. A lot. Ha-ha.
The statement is made in the second article down (right now) called "Bridging the Gap", on the Google AdSense Blog - [adsense.blogspot.com
It should be OK to link to that, I hope, since it is THE authoritative reference site.
[edited by: engine at 1:05 pm (utc) on Apr 26, 2010]
[edit reason] Fixed link [/edit]