Memoir '44
When I Counter-Attack, do I copy the opponent's unit choices, number of units, or dice?
None of those. You play the copied card as your own: choose units from your own army up to the card's limit, and roll any dice yourself (for example Their Finest Hour counts your own hand). If the card orders a unit type and you own that type, you must order it even if your opponent used the one-unit-of-your-choice fallback. Section cards are mirrored, so a Right Flank card becomes a Left Flank card for you.