What is the minimum number of domain controllers you should configure to ensure that a domain is fault tolerant?

It directly depends upon following things and u can think of more. 1. Organization requirements towards security ( intend of your policies and procedures) 2.Oraganizatioin maturity level 3.Application dependencies 4.Availability of service

Really this depends on the szie of your domain. What might be overkill for one domain, might be very sparse for another.

Another big factor is the geographic area of the network. If you had a big office here and a smaller office down the road in a different building, you'd probably be aswell to put a DC in the small office even if you did only have 4 or 5 computers in it, you could even make the DC a global catalogue. But no matter how many DC's you have. A WAN is never truly going to be fault tolerant. For example the sql slammer worm a few years back took down hundreds of WAN's.