There are more than 20,000 Uber drivers in San Francisco

"At Uber we are thrilled to be playing a role in unlocking economic opportunity and empowering entrepreneurs across the Bay Area, and we are even more excited to be able to accomplish this at such a large scale.
Today we’re proud to announce that we reached a new milestone: The number of Bay Area driver-partners on the Uber platform exceeded 20,000 for the first time… and we were not even halfway there just one year ago."

Source:  Uber blog, 14th April 2015