This is from a UK jar ops manual. (from memory though!)
Cat 1 - None, met viz can be factored to give equivalent RVR.
Cat 2-Cat 3B - Theoretically one, if stop end is not avail or taken as advisory , and either mid or t/d is unserviceable. Usable configs are:
t/d mid end
Serv Serv not req
Fail Serv not req
Serv Fail not req