Based on three different estimates, and admittedly sketchy data, it is plausible that more than 20,000 US companies have legal departments. Test my reasoning and let me know how well it holds up.
First, In-house lawyers as percentage of practicing lawyers, and then average lawyers per law department. Lawyers in the Us held about 759,200 legal jobs in 2008, according to the US Bureau of Labor Statistics website. http://www.bls.gov/oco/ocos053.htm Of them, what percentage practice as an employee of a company? An unpublished article by Michele DeStafano Beardslee and three co-authors, discussed at the Georgetown Conference on the Future of Law Firms at 6, n. 22, tells us that in the United States, “in-house counsel, as a percentage of all lawyers, was declining from 11% in 1970 to 8% in 1995, where it remained through 2005” (citations omitted).
Assuming the percentage has held since then and something like 800,000 lawyers practice law in the United States right now, an eight percent figure yields 64,000 in-house US lawyers (See my post of Sept. 25, 2005: ACCA estimate of 71,000 non-governmental in-house lawyers.).