question about magic pen

There is no cap on armor or magic pen. You can get as much flat pen as you want (no soft cap, no limit).

There is no cap on % pen either, but it does not stack additively. It stacks multiplicatively.

For example, if you have the % mastery (6% magic pen) and void staff (35% magic pen), you would have 39% magic pen.

(1-0.35) x (1-.06) = 0.611. That's the percent damage they would take. So you would have 39% magic pen with those two items. Then if you had flat magic pen, that would be factored in after the percent is factored in.

For example, if you had those two items and boots, and your enemy had 100 Magic Resist, they would effectively have 46 magic resistance.

There is not a hard cap on penetration, no. The items that give penetration and reduction have UNIQUE qualifiers, though. You could stack Sorcerer's Shoes, but that would be an utter waste of slots.

There isn't an hard cap, but ionization choices limit it. You get 6% from mastery, but you can also get runes, and items like

thank you all for your comments. The reason I ask, and I dont know if any of you have looked at this, but on the in game statistics page it seems to cap at 6%. The magic penetration can go up but the percentage never moves. And im not stacking unique passives either. I'm curious to how there isnt a cap yet 6% seems to be the maximum pen allowed.