A numeric norm undoubtedly exists between the number of cases handled per litigation lawyer inside and the number of outside counsel managed by the same lawyer. In general, you would expect that the more lawsuits in the dossier of an in-house lawyer the more law firms that lawyer would manage. If the lawyer had on his or her plate only one massive case, it is quite likely that only a handful of law firms are involved. At the opposite extreme would be a caseload of hundreds of product liability cases each handled by a different local law firm.
I have written about litigation case loads of 20-30 per lawyer as about the maximum for effectiveness (See my post of April 8, 2005: from a survey about 50 cases per litigator.) and I have written about 5-7 law firms per in-house lawyer as a rough guide (See my post of July 17, 2009: law firms and vendors per user; and Feb. 11, 2007: in smaller departments, about three law firms hired regularly per in-house lawyer.), but I have seen no metrics that combine and describe the incidence of the two figures.