This is the 1555th most frequent Portuguese word.
nº
"nº" is an abbreviation for "número," meaning "number" in English.
In this sentence, 'nº' is used as an abbreviation for 'número' to denote the apartment number in an address.
O apartamento nº 23 é onde eu moro.
Apartment no. 23 is where I live.
Here, 'nº' indicates the specific number of the book within a collection, acting as an identifier.
Você tem o livro nº 5 da coleção?
Do you have book no. 5 of the collection?
The 'nº' signifies identification number of a specific item, in this case a document, used for cataloging or record-keeping.