Purge -- all at once or staggered?

[HPAVC]

I can't seem to figure out if when I cast purge on a target (with all 4 shamans) it removes up to 8 debuffs or just 2.

I mass purge with a key and it removes 2 magic debuffs for each cast for each shaman.

I used the pvp area in DM while I was there, dropped paladin and the shaman purge spammed away. A few issues I saw were that a 'resist' occured and that seemed to just remove one of the two buffs, partial resist.

Fwiw the same key modified also mass wind shears. I usually just switch from button mashing dps key to mashing that key and it works fine. Not all the shaman will be able to cast it since some will be still dpsing at different rates.