Drum Rolllllll Please! After an attempt at a comprehensive study, the answer is 441,541. Streetsblog has a good story about how even making this attempt is groundbreaking. Most cities answer is always Not Enough! Even if their town could be mistaken by pilots for 1945 Dresden.
Should it be a surprise, people waste and demand more and more of something if it's given away?
"Using data from the MTA's Transportation Fact Sheet, Weinberger noted that despite 28.5 percent of San Francisco households not owning cars, "enough households have multiple vehicles that the city's population, collectively, owns over 8 percent more vehicles than households."
"As we all know, the more parking there is available, the more convenient car use becomes relative to other travel options," said Weinberger. "The more convenient car use is the more likely a car will be used."
Shoup marveled at how much parking in San Francisco is free, especially when compared with the price of housing. "One surprising result is that 72 percent of all the publicly-available parking spaces in the city are free," he said. "In San Francisco, housing is expensive for people but free for most cars."
Todd Litman, the director of the Canadian think-tank Victoria Transport Policy Institute, said the census showed that "in many situations there is not actually a shortage of parking spaces, rather, the available spaces are not being well-utilized." Litman said the solutions were parking management strategies such as more car sharing, efficient pricing, and parking cash out, which "can address parking problems in ways that also help achieve economic, social and environmental objectives."