In the spirit of strange questions google might ask...something to get you thinking a little bit.
How many superheroes are needed in San Francisco?
Let's assume that the superheroes are needed to fight crime. All of it. In that case you'd need as many superheroes as it would take to stop all the crime in the city.
If you look at the FBI's Uniform Crime Report for 2009 you find that San Francisco had ~38,700 crimes in the first 6 months of 2009. That is ~215 crimes per day.
If you assume that all crimes occur at night (say between 8 pm and 5 am), that is ~2.5min per crime.
If you assume that the superhero instantly knows when and where a crime occurs and takes 1 min per crime to stop the evil-doers, that leaves ~1.5 min to travel between crimes.
If you assume that the superhero can fly as "fast as a speeding bullet" and that the bullet in question is from a 45cal Colt Revolver, that's 661 mph or ~11 miles/min. So in 1.5 minutes the hero could go ~16.6 miles (without even a sonic boom to wake people up).
San Francisco land area is roughly 7 miles x 7miles, which means that one superhero could easily cover all the crimes in the city.
So the answer is: One superhero.