how many buildings were knocked down in the San francisco earthquake?

Answer 1

Many. The exact number is unknown. But if you are refering to the 1906 earthquake, then the greatest damage came from fire.

Answer 2

In the 1906 Earthquake, most buildings in San Francisco held up. People even started to go about their day afterward.

The fire was the problem. Since the quake had ruptured most waterlines, firefighters had nothing with which to combat the blaze.

So they started backfires, they dynamited buildings so the fire couldn't leap one building to the other. The did this all through the night in one of the largest and most misguided fire fighting efforts undertaken in any city.

The city looked like it had been bombed. Newspapers ran headlines like "Earthquake and Fire, San Francisco in Ruins" and they weren't exaggerating.

Jack London toured the scene afterward, and wrote many pages about the devastation he saw there. Only the Pacific Heights area was left. The rest of the city was annihilated.

Here is a link to a personal account of the days following the earthquake. It's fascinating reading.

http://www.johnmartin.com/earthquakes/eqpapers/00000047.htm