Does Master of the Wild Hunt deal damage multiple times?
The text states that each wolf deals damage. This implies that there are multiple damage sources.


This is correct, but it's not relevant.

If you used Master of the Wild Hunt's second ability targeting an Unbreathing Horde, at most one counter would end up being removed no matter how many wolves you have.  All the wolves deal their damage simultaneously, so the Horde is only dealt damage once, even if the damage comes from multiple sources. Unbreathing Horde's replaces the receiving of damage, not the receiving of damage by a source.

Compare

"If Unbreathing Horde would be dealt damage,"

with

"If Unbreathing Horde would be dealt damage by a source,"

Phantom Nishoba has a ruling confirming this.