Take a look at this:
http://www.census.gov/econ/industry/geo/g711130.htm
The total number of musicians employed at musical "establishments" (presumably, studios, labels, etc.) in the US is 41,000.
As for the total number of professional musicians in the US:
Musicians, singers, and related workers held about 240,000 jobs in 2008, of which 186,400 were held by musicians and singers; 53,600 were music directors and composers. Around 43 percent worked part time; 50 percent were self-employed. Many found jobs in cities in which entertainment and recording activities are concentrated, such as New York, Los Angeles, Las Vegas, Chicago, and Nashville.
http://www.bls.gov/oco/ocos095.htm#emply
Out of 150 million+people in the US labor force, around 1/6 of 1% held a job as a musician in 2008.
Rather sobering, no?