Do you need to know how many librarians, doctors, lawyers, microbiologists, human resource officers, etc. are employed in the United States or a specific state? If so, the Occupational Employment Statistics (OES) database from the Bureau of Labor Statistics can likely give you what you are looking for. The data includes the total number of people employed by occupation (does not include self-employed workers). For each occupation, you will find hourly and annual wage percentiles as well.
Hint: If you are looking for multiple occupations (as in the example image to the right), use the Cntrl button and click to make multiple selections. Also, the BLS has a wonderful Help Page in the top right of the database page.