I might be wrong but it seems like the api is creating one shipment for each package with in an order. I assume that mean you will get a tracking number for each package (Unless tracking number is associated with order, not the shipment in the api). The Scenario I have seen this is not something desirable. UPS[0] and Fedex[1] I believe let you ship up to 20 boxes in one shipment.

[0]https://www.ups.com/content/us/en/resources/sri/glo_mlt_pc.h... [1]http://images.fedex.com/ca_english/businesstools/shipsoftwar...