The definitions of "correspondent account" and "account" provided in the BSA regulation don't give much helpful guidance on this.
"Account" is defined a "formal banking or business relationship established to provide regular services, dealings, and other financial transactions" -- so if you only confirm 1 LC that was issued by a particular foreign bank, should that be interpreted as automatically meeting the definition of an account? The reference in the definition to "regular services, dealings", etc., suggests that the frequency of those types of transactions may dictate what is/isn't considered to be an "account".
The conservative approach would be to treat all such transactions as being subject to Section 312. However, it might be acceptable to set a minimum threshold on the number of such transactions occurring within a particular timeframe to determine what will be considered an "account" subject to Section 312 -- and document that in your procedures.
I would suggest that you have a discussion with your regulator about this to get their buy-in on how you propose to interpret this.