We might be able to deduce the approximate number of legal departments in a country based on its Gross Domestic Product (GDP). Follow my reasoning from four examples.
Portugal’s organization for in-house lawyers has 1,200 members. If the typical law department employs three lawyers, that suggests something like 400 departments (I do not know that every Portuguese in-house counsel belongs.). The country’s GDP in 2008 was approximately $243 billion, which leaves 0.6 law departments per billion.
The United States, at $14.2 trillion in GDP and 20,000 or so law departments, supports around 0.71 departments per billion of GDP (See my post of April 30, 2010: three indicators to around 20,000 departments, and James Merklinger’s comment in support.).
England and Wales have about 20,000 solicitors working in the employed sector, according to a fact sheet by The Law Society. Again assuming three lawyers per department on average, the 6,600 legal departments supported a UK GDP in 2008 of about $2.6 trillion (which is larger than the England and Wales GDP). Hence the figure of 0.4 law departments per billion undershoots somewhat for the entire UK. Let’s grab at 0.5.
Using the same assumption of three lawyers for the average legal department, Belgium may have something like 420 law departments (See my post of Feb. 16, 2010 #2: mandatory to belong to Belgian in-house group, which has 1,300 members.). With a GDP of approximately $497 billion in 2008, that leaves 1.2 law departments per billion of Belgian GDP.
All the GDP figures and the conversion of the figures into US dollars come from the website of Trading Economics.
To summarize, the four developed countries above (Portugal, USA, UK, and Belgium) produce estimates of legal departments per billion dollars of GDP at 0.6, 0.7, 0.5, and 1.2. The results, being clustered reasonably closely, support a hypothesis that for developed countries there is a plausible and fairly narrow range of law departments per billion dollars of gross domestic product.