+12 Armor pen is enough for the jungle. It makes it so you can do true damage to the buffs. And iirc, blue and red only have either 12 armor of 12% armor resistance. Also since you can't go lower than 0 with runes and masteries, it would be a waste to have more armor penetration. It works for champions I guess but I wouldn't recommend it.
A generic jungle would be:
-8 Flat AD marks
-1 Armor penetration
-9 Flat armor seals
-9 Flat MR per lvl glyphs
-3 Armor penetration quintessences
In some cases, you can replace 8 Flat AD with 8 Flat Attack Speed. Or replacing MR7lvl glyphs with glyphs of your choice (CDR for example)
All in all, it all depends on how you jungle, I have seen people just not get any ArmPen at all and get +15 AD (9 AD Marks 3 AD quints) and do well, I have also seen people replace MR/lvl glyphs with attack speed glyphs and do well. It all depends how you play, and who your character is. (For example, Fiddles should never be jungling with attack speed runes like Skarner)
If you are more of a novice jungler, and don't know your character that good, I suggest you just go with the generic rune placement I wrote.
EDIT: The new jungle is pretty !diot safe anyways, the only mandatory runes are Flat armor seals