“There are 30,000 companies in the United States that have at least 100 employees,” it says in Intellectual Prop., Fall 2010 at 48. That might be another clue as to how many law departments exist in the country, although presumably a smaller number than 30,000. The median number of US attorneys per 1,000 US employees in large law departments is 1.5 (See my post of Jan. 27, 2006: lawyers per 1,000 employees.). If that metric holds even approximately among the 140 million people over 16 employed in the United States (see the Bureau of Labor Statistics website) that would suggest something like 200,000 in-house lawyers. At 2-3 lawyers per average legal department, that would lead to 70-100000 departments, which is a very crude (and much too high) directional pointer.
Earlier posts speculated on various bases for estimating that the US has something like 20,000 corporate legal departments (See my post of Sept. 25, 2005: ACCA estimate of 71,000 non-governmental in-house lawyers; June 15, 2009: almost one out of five lawyers in a large survey had gone in-house by their seventh year of practice; March 2, 2010: presumed even distribution of number of departments across industries after adjusting for size of industry; April 30, 2010: demographics, corporate revenue, and law firm fees; May 11, 2010: GDP-based calculation; and May 12, 2010: publicly traded companies and corporate tax returns.).