The prime source she uses for the incidence of surgery is The Incidence and Prevalence of SRS among US Residents(PDF) by Dr Mary Ann Horton. She used a simple technique - she determined that most such procedures are performed by a comparative handful of specialist surgeons. It's not exactly a common form of surgery, nor is it simple. Getting it wrong, even in a minor way.... let's just say that trans people take very great care indeed in choosing which surgeon to go to. As should anyone contemplating surgery on their "naughty bits". And in order to qualify for surgery, one of the hoops that must be jumped through is convincing the gatekeeper psychs that you've done the research in this area, and know the Pros and Cons of different surgical techniques.
The number of US residents undergoing primary SRS in the year 2001 (run rate) was 1170 (740 Male to Female or MTF and 430 Female to Male or FTM.)Some extra work is needed to take into account "re-do"s of surgeries which would decrease the number, then add some more to take into account the lack of response.
Persons desiring irreversible surgical procedures who value the quality of the result usually go to a surgeon who has performed the procedure many times previously. It is known within the American transgender community that the vast majority of transsexuals seeking surgery go to one of a relatively short list of surgeons for their final surgery. By reputation, the vast majority of US transsexuals went to one of 15 surgeons. (Eight of these surgeons are in the US, and seven of them are not.) This study refers to these 15 surgeons as major surgeons. It is believed that the major surgeons account for nearly all the primary surgeries performed on US residents.
...12 of 15 major surgeons, or 80%, provided usable data for this project. Two surveys were returned by surgeons who were not on the list of major surgeons. One of these provided MTF data for the study; one provided both MTF and FTM data.
The 14 valid surveys (a 25% usable response rate) represented 866 MTF primary surgeries performed by 10 surgeons, and 336 primary FTM surgeries (top surgery) performed by 10 surgeons. 7 of the 14 surgeons performed both MTF and FTM surgeries, 3 MTF only, and 4 FTM only. (Of the 12 major surgeons with usable surveys, 3 did MTF, 3 did FTM, and 6 did both.) Missing data was reconstructed and extrapolated, as described above, to arrive at a total annual run rate of extrapolated total primary surgeries.
Call it 1,000 per year. I would have estimated more than that, but those are the figures.