Formal:
* A consignment of boxes
* A shipment of boxes
* A stack of boxes
* A pile of boxes
Informal:
* A bunch of boxes
* A heap of boxes
* A load of boxes
Creative:
* A nest of boxes (if they're arranged in a specific way)
* A fortress of boxes (if they're used for protection or play)
* A symphony of boxes (if they're arranged in a visually appealing way)
Ultimately, the best collective noun depends on the specific situation and your desired tone.