---How much is necessary?

-Just quints with Attack Speed marks...or All armor pen?
-or a mix of marks..?

Nevermind, just tried my Jungle Malphite out for the first time...

-I decided that the new jungle is too easy for me to care about Armor Pen...

+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

Blue/Red are 20 armor
Dragon is 21

http://leagueoflegends.wikia.com/wiki/Monster

There you got more info about the jungle.

Armor Pen Marks are never bad decision to get for jungling. They scale better late game for damage then AD marks do and quints.

My jungler normally has movement speed quints to get from place to place faster. Marks kind of depends on the jungler. Like Skarner and shyvana I take attack speed marks instead of armor pen to take advantage of skarner's cooldown reduction per hit and shyvana's. allows you to spam your Q that much more on both champions.