There's no limit in the number of Worm Counters that can be placed on "Worm Illidan".
When a monster or Spell/Trap Card is Set, or a monster is changed to face-down Defense Position by the effect of "Book of Moon", 1 Worm Counter is placed on this card.
The "You can remove 2 Worm Counters from this card to destroy 1 card your opponent controls." effect is an Ignition Effect. (It targets 1 card your opponent controls. Removing 2 Worm Counters is a cost to activated this effect.)