Fiends of Slaanesh special rules question

Hi guys, I have a unit of nicely painted Fiends of Slaanesh collecting dusts. Would like to play them once awhile but they have very confusing rules.
Would seek the rule lawyers here for help. Below are the actual rule wording in the codex:

1. Disruptive Song: Enemy Pyskers within 12" of a Fiend of Slaanesh have a -1 penalty to their Leadership when taking Pyschic tests.

So the argument....

1. Does the penalty stack? If yes, is it per model basis (eg: there are 6 Fiends in a unit in range, so -6 Ld) OR is it per unit basis (eg: there are 2 units of Fiends in range, so -2 Ld)

What do you guys say?

Not sure lah.. usually they say "... in range of one or more ..." or "... for every ... in/within ..."

Does it stack? maybe..