Mega-cities probably house the largest law departments and almost certainly the largest share of them
Of the Fortune Global 500 companies, 165 are based in 10 vast cities. I presume that their legal departments staff many of their lawyers in those megalopolises. Here are the leaders from Foreign Policy, Sept./Oct. 2010 at 125: Tokyo (51 Global 500 companies and probably law departments), Paris (34), New York (26), London (21), Seoul and Chicago each with 11, Hong Kong and Los Angeles and Sydney with three each, and Singapore (2). Regrouped, Asia accounts for 70 of the 165 companies and presumably huge legal departments, Europe boasts 55 of them, while the three largest US cities combine for 40.
The General Counsel Metrics benchmarks look at law departments of industries, countries, and regions. Perhaps in the fullness of time there will be a breakdown by cities.