box number is a noun meaning "the number of a letter box at the post office where mail is collected". It carries 2 recorded senses.
the number of a letter box at the post office where mail is collected
a mailing address to which answers to a newspaper ad can be sent
Definitions, synonyms and semantic relations for box number come from Princeton University’s WordNet. Commonness is measured against a corpus of everyday English. Full sources and licences.