The total revenue of the Fortune 500 in 2005 was $9.1 trillion. Based on the Hildebrandt 2006 Law Department Survey, median lawyers per billion dollars of worldwide revenue the same year was 3.1 (per billion dollars of US revenue it was 4.4 lawyers). Multiplying the Fortune 500 revenue by the worldwide ratio indicates about 28,000 lawyers were employed in-house by the Fortune 500.
We might take one more step and assume that 90 percent of the lawyers who work for Fortune 500 companies are admitted somewhere in the United States (See my post of March 19, 2006 to back that estimate.). That number, about 25,000, should be compared to the 72,000 corporate lawyers identified by ACC in 23,540 companies circa 2004 (See my post of Sept. 25, 2005 on ACC data for 2004.).
It makes sense that more than a third of the in-house corporate counsel in the United States are in the huge companies, the top two percent of the ACC group.