Regular mobs have around 9k resistances, bosses have around 18.5K resistances (I'm sure someone can give you the exact numbers if I'm off.)

The most common taunt, Pierce Armor, grants Major Fracture/Breach which reduces physical and spell resistance by 5280.

Without checking, I think my stam sorc has around 8-9K penetration while wearing Spriggans.

Are we discussing Physical Penetration or Spell Penetration? Personally run between 1.9 k- 2k Physical Penetration on my Stam NB and Stam DK solely from Champion Points.

If you wear light armor you will have an innate 4884 from the light armor passive. You want 9k pen period. So you have 4884 plus cp about 40 points should get you close to that or run the lover mundus and put cp elsewhere. The tank will be running ransack or pierce armor which is roughly 5k then the crusher enchant is almost 2k and alkosh armor set is about 3k. So between you and the tank will achieve 18k penetration. If solo you want to shoot for 4884 from the passive..40cp in spell erosion...lover mundus and spinner 5pc then a damage or sustain set as needed. This will get you pretty close to pen cap for boss fights. Penetration is situational. If overland fights with no boss go 9k same for VMA adds and some bosses but most bosses there have 18k resist. Group dungeon 9k. Solo world bosses and pvp around 12-15k.