If my only Eternatus VMAX with "Eternal Zone" Ability is Active and gets KO'd, do I reduce my bench size down to 5 Pokemon before or after promoting one to the Active Slot?
You discard down to 5 Benched Pokemon before promoting a new Active Pokemon in this scenario.
Source: TPCi Rules Team (2020-08-20)

Relevant cards

Note: All applicable cards may not be listed below.
SWSH3_117
SWSH3_117
SWSH3_192
SWSH3_192