I lived there for 15 years and didn't see all that many. But I lived in Parkland, which is a bit north of the drug dealers and a bit south of the retired businessmen.
Parkland is at the southern edge of the first Bentley Belt, which extends from West Palm Beach to Boca. I've seen at least six Bentleys in Boca. The only other place I've ever seen one was curbside at LAX. I've never seen a Bentley anywhere in the Bay Area.
Fareed's comment is weird. Where is there a parking garage on Sand Hill Road?
I work on Sand Hill and there's very very few garages around here. Just lots of open parking lots all over the place.
Also the culture here is to leave the fancy cars at home and drive a high-end (but not exotic) german car to work. In fact, tons of guys drive Priuses. Not too many bentleys - it'd be considered too crass.