You are most welcome.
Your friend's number of 6,000 is quite unlikely. Here is a quick search for S.F. demographic; says it's for S.F. and S.F. county. The total population is around 744,041 in 2006.
let's say we have 6 people per SFR on the average, 6,000 homes are for 36,000 people. Of course, depending on how small a section your friend is talking about.
Just think about how many areas there are in S.F - Alamo Square, Anza Vista, Aquatic Park, Balboa Terrace, bay View, Bernal Heights, Sunset, Richmond, Castro, Chinatown, Pacific Height, SOMA, etc.
Knowing how far the number might be from 6,000, are you still interested in finding out about the number of SFRs in S.F If so, which specific area(s) are you interested in?
Although keep in mind that S.F. Bay Area real estate is very location driven (for that matter, any real estate), so really depends on which area /characteristic you are interested, that's where you might want to put your emphasis in.