The requirement is to manifest packages at the “lowest external packaging unit” per 19 CFR 4.7a(c)(4)(v).

The example in the regulation is “…a container containing 10 pallets with 200 cartons should be manifested as 200 cartons…”

If you have 1,000 house bills, and each of those house bills contains only one item, then each house bill would be for “one carton”, “one bag”, or similar, even if all of those house bills are in a larger box.

If a single house bill contains “sunglasses, neck ties, shirts” and they are all in one box, then it is ok for that to also be “one carton”.

If the house bill has those same items, but they are all packaged separately, then that single house bill would use “3 cartons” or “3 bags” etc.