how many professional musicians are there in the United States?

Dear Vizier, 

According to official US government information: "Musicians, singers,
and related workers held about 249,000 jobs in 2004. Around 40 percent
worked part time; almost half were self-employed."
(SOURCE: Bureau of Labor Statistics, U.S. Department of Labor,
Occupational Outlook Handbook, 2006-07 Edition, Musicians, Singers,
and Related Workers, on the Internet at
http://www.bls.gov/oco/ocos095.htm  (visited September 14, 2006).).

I hope this answers your question. Please contact me if you need any
clarification on this answer before you rate it. My search terms:
professional musician career site:gov