You’ll get the last skeleton at 20/22 Undead Legion: +1 for every third level till softcap (12) and +1 for every fourth level after softcap.
The main problem is not the legion, but the main skill: you 100% need either gloves or a helmet, otherwise even with +3 on the MI medal you will only get 24/26 (or you will have to take a neck and a relic for +1 to Necro for 26/26) to the main skill.
As one of those who took part in the PT, and who created several builds based on this weapon (in the middle of the test, even before the final buffs-nerfs - any Ravager in less than 30 and even less than 20 seconds) I perfectly understand the reason of this nerf. With +2 Necro and playing Ritualist, you’ll get about 30 free skill points and can take Skeletons, Primal Bond, Primal Spirit (or Reap Spirit with +3 
) and Briarthorn with taunt and aura + Mogdrogen Pact/Blood Pact at the same time. All this together, especially after some buffs, was a huge-game-breaking-damage-boost.