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%.
It's not possible to reach 100% damage reduction.
Effective HP (as a percentage of 'base HP') is linear with respect to resistance and armor (they're calculated separately, then multiplied). At level 60, every 3000 armor (or 300 resistance) gives you +100% EHP.
It may be possible to reach 95% reduction just from armor, but it's not a good idea. You'd need 57000 armor (which probably isn't possible even with BiS +armor gear). However, to get 95% DR from resistance and armor combined, you need about 10400 armor and 1040 resist all, which is nearly 1/3rd the item budget.
See this D3 forum post for much more detail.