According to MC Wiki (So reliable I know) the max value of generic.armor is 30 (http://minecraft.gamepedia.com/Attributes#Attributes_available_on_all_living_entities). However, if you test this in game on yourself (Using custom items), you will find that not only does Armor visibly cap at the same place it used to, 10 bars/20 points, you take the same damage with 20 Armor as with Armor that grants 30 Armor.

You can test this by giving yourself two chest plates, one with +20 Armor and one with +30 Armor. Summon a mob that has generic.attackDamage base of 60 (Or some other high number) and give yourself a lot of Health Boost. Equip the 20 Armor chest plate and note the damage you take. Heal and repeat with the 30 Armor chest plate and you will find you take the exact same amount of damage.