The place you want to head to is the website for the American College
of Obstetricians and Gynecologists at:
ACOG educates and represents the interests of OB/GYN's in the US,
virtually all of whom are members.
In the upper left-hand part of the ACOG homepage is a link to "Find a
Physician". Immediately underneath it says: "40,000 Ob-Gyns".
So that answers the first part of your question -- there are about
40,000 OB/GYNs in the US. On their "About ACOG" page:
the mention that they have 46,000 members, though that number may
include some non-physicians as well.
To narrow it down by city, click on the "Find a Physician" link, and
press "Accept" on their terms of service page, which will take you to
a map of the US.
Click on a state of interest, which will bring up a list of cities in
the state. Clicking on a city will bring up a list of names of all
the OB/GYNs in that city.
GOOD TRICK TO KNOW...You can also search by zip code. Entering a
*partial* zip code will return a much larger list of physician names.
For instance, searching on 11210 returns all OB/GYNs for that zip code
in Brooklyn, but searching on just 112 returns a larger list of names
all over Brooklyn, and searching on just 11 returns an even larger
list of the entire Brooklyn-Queens-Long Island area.
You might also want to have a look at a question that --
coincidentally -- I answered earlier today on a similar query, which
also describes the resources available through the American Medical
This should provide all the information you need.
Before rating this answer, however, please let me know if you need any
additional clarification, or additional information on this topic.
search strategy: Used bookmarked sites on medical resources.