It seems that the damage reduction curves for armor and resistance gain are not linear, so at some point it'll reach a limit. What is this limit for armor and resistance? How much armor will I need for 95% reduction (just counting armor reduction, not from anything else such as the barbarian/monk passive reduction of 30%). Is it possible to have enough armor for 95% reduction?
There is an upper bound for damage reduction... it's 100%.
See this D3 forum post for much more detail.