Can you unhide multiple hidden cards at the battleground being attacked, and can you also unhide cards on other battlefields (that aren't being attacked) to buff your units?

Ruling: Yes, you can unhide multiple hidden cards at reaction speed, including cards from battlefields that are not being attacked. Each unhide is a separate reaction that can be chained together.

Nuances:

• When unhiding a card from a different battlefield (not the one being attacked), any targeting choices will be restricted to only that battlefield where the hidden card was located