The ALM benchmark survey, released in September, covers 99 companies. The average number of law firms employed in 2010 by the subset of those companies earning between $1 billion and $4.9 billion was 43. Very roughly, that revenue range typically corresponds to 5-25 lawyers in-house. If 10 were the mid-point, the average would come down to a bit more than four law firms per in-house lawyer.
This blog has no clear formulation of law firms per inside lawyer (See my post of Feb. 11, 2007: rule of thumb of three primary firms per in-house lawyer; July 17, 2009: law firms and vendors per user; April 16, 2010: rough estimate of two firms per lawyer; and May 12, 2010: six outside law firms per inside lawyer at Cox Communications.).