The document frequency is the number of documents in which the word or phrase appears.
If the corpus has 100 documents and 2 documents contain the word city:
document number 7 contains 17 instances of city,
document number 31 contains 6 instances of city,
the document frequency of city is 2, because 2 documents contain the word.
It is not important how many documents the corpus contains or how many times the word appears in total.
The document frequency can be better suited for comparison in situations when the corpus contains a small number of documents with an extremely high frequency of particular words.