I have igni with intensity ubove the 75% cap for armour reduction.

when using it for its armour reduction effect how many times should i hit each enemy with it?
and does it work aginast all enemies or just ones who have visible armour.

The short answer is you must hit them repeatedly, though how many times depends on the quality of the armour. The answer to your second question is it has NO effect against unarmoured foes like drowners etc. It is an *armour* debuff.

IN that case
How do you know which foes have armour (other than seeing Armour)

In general, if you don't see it, ie. actual armour, or natural armour (shell-like protection), then they don't have it.