We have some idea, actually, insofar as the number of trans people who get GRS is much smaller than the number of total trans people (the procedures are quite costly, often not covered or completely covered by health care providers, seldom available without travelling long distances and rarely performed on patients less than 18 years of age, or with certain medical contraindications). The number of people who've actually had SRS serves as a very crude lower limit against which you can check other numbers and get some idea of prevalence.