According to the Bureau of Census Statistical Abstract of the United
States, as of December 31, 1998, there were 777,900 Medical Doctors in
the United States (including Puerto Rico and outlaying areas).
In addition, there were 40,800 Doctors of Osteopathy. See the table
at: http://www.census.gov/prod/2001pubs/statab/sec03.pdf and scroll
down to frame 19 (which is page 123, Table 186).
In addition, the Statistical Abstract provides quite a bit of
amplifying detail, for example:
Professionally Active: 707,000
U.S. Educated: 533,400
Foreign Educated: 173,400
In 1980 there were 467,700 M.D.s,
1985 there were 552,700
1990 = 615,400
1995 = 720,300
1998 = 777,500
In addition, the information is broken down by specialty. Neighboring
tables include data concerning nurses and other professionals employed
in health care professions.
I hope that this gives you the data you desire. If there is any
amplification, PLEASE ask for clarification.