The proper ratio is 1.34 current (incumbent) firms to 1 new hopeful.
Seriously, no research or survey data has come to my attention that answers the question. To say, “It depends,” is glaringly obvious and obviously useless. Even to list some of the considerations adds nothing to what experienced in-house counsel already know.
My precise ratio is silly but still my point is sound. The arguments for inviting somewhat more incumbents than newcomers are that (1) devils known are better than devils unknown, (2) you still want a horse race and legitimate contenders who feel they have a shot at the prize (See my post of Sept. 3, 2006 on the false perception of fixed selection processes.), (3) you are damning yourself and your management of outside counsel if you jettison most of your currently-serving firms, and (4) it takes more time and effort to learn about firms that are unknown to you.