It's really bugging me that, as written, HOLD GROUND works if you have any one copy of the card with a single unrevealed marker on that card, regardless of what's on the other clones of the card. The intent was to count the markers on
all associated cards, but in this context (especially with the noun in the singular), the "any" that was used actually communicates the opposite of "all" ... it is short for "any one."
Here's one way to align the text with the design intent:
HOLD GROUND
If there is only one unrevealed Order Marker among all 20th Maine Volunteers Army Cards you control, all 20th Maine Volunteers you control add 2 to their defense dice.
"Among" is a good tool for this job, but I expect that some may dislike it; so here's another option for consideration:
HOLD GROUND
If there is only one unrevealed Order Marker on the 20th Maine Volunteers Army Card(s) you control, all 20th Maine Volunteers you control add 2 to their defense dice.
I also expect that some will complain about the parentheses, but this is a situation where they are appropriate, and Hasbro already did it on
Tul-Bak-Ra (three times, in fact).
If you
really want to follow the "Dünd standard" (but please don't!), it might go something like this:
HOLD GROUND
If there is only one unrevealed Order Marker on the 20th Maine Volunteers' army card (or cards if you have more than one common card for those figures) you control, all 20th Maine Volunteers you control add 2 to their defense dice.
Whatever wording you go with, the reference to the card title should be corrected in the card image and PDF (the OP is already correct). Heroscape convention allows toggling between singular and plural when talking about
figures, but when referring to the army card itself the title should be exact. (There is no such army card as
20th Maine Volunteer.)