For law departments with more than, say, 15 lawyers, the median ratio is three lawyers for every paralegal. What that median conceals, however, are wide disparities among different practice areas.
My consulting experience suggests that the highest ratios of lawyers per paralegal are in tax and M&A. Somewhat more paralegals per lawyer would likely be found in commercial, transactional groups of lawyers – and even more so where paralegals administer contracts – and in litigation, if the law department handles much discovery work. The lowest ratios of lawyers to paralegals, even to the point where paralegals outnumber lawyers, are found in such document-intensive practices as real estate, corporate secretary, and immigration (See my post of Aug. 21, 2005 on the differences, if any, between paralegal and legal assistant.).