There's a scaling law as regards city size and number of industry and service types (I forget what the exponents are) but there's no good theory behind the empirical relationship. People have made stabs at it -- including Krugman, when he isn't writing New York Times editorials -- but it's a little intractable.

If memory serves, it's related to calculating the optimal number of firms in an industry to maximize welfare. Also a little intractable.

Carlos