19 CFR 141.51: Quantity usually required to be in one entry.

All merchandise arriving on one conveyance and consigned to one consignee must be included on one entry, except as provided in § 141.52. In addition, a shipment of merchandise that arrives by separate conveyances at the same port of entry in multiple portions, either as a shipment split by the carrier or as components of a large unassembled or disassembled entity, may be processed under a single entry, as prescribed, respectively, in §§ 141.57 and 141.58.