There are more than 20m Tumblr blogs

"According to their respective websites, 4-year-old microblogging platform Tumblr now hosts more blogs than 8-year-old WordPress.com.
In January, Tumblr had more than 7 million individual blogs. At the time of this writing, the total blog ticker on its site read about three times that at 20,873,182 — beating out WordPress.com’s current count by about 85,000 blogs."

Source:  Mashable, 15th June 2011