I'm not sure if I've understood your question so apologies if what I'm about to say is irrelevant.
Henchmen groups act like a single figure so they only attack, etc once. If they are attacked they defend like a single figure, so you roll the number of dice equal to DR as usual, not once for each henchman, but instead of removing vitality points if you fail to match the DN you remove one henchmen model if it was a ranged attack and henchmen equal to the vitality a single character would have lost if it was a close combat attack.
There are some exceptions to the above. A henchman group attacked by multiple characters or groups splits up to fight them and behave like seperate groups, so could attack more than once. Ranged weapons with radius for example can also kill more than one henchman in ranged combat.