The phrase "one in the box" means having a backup or spare item, usually in case the original one gets lost or doesn't work.
Sentences with «one in the box»
A great idea, but you'll need to buy a wireless charger separately as one is not included in the box of Mod nor any Moto Z - series device. (pocket-lint.com)
Each set comes in a box with a slot for your business card or personal note. (realtormag.realtor.org)
Each strength comes in a box of 6 tablets. (vetiq.com)