Hello Will, "a" and "the" are both articles to define nouns. "a" is an indefinite article which means it doesn't point or define a specific person or thing belonging to a noun group. "A number" of things or people means that there are several of them . " He decided to interview a number of candidates today". That refers to several candidates ,whose exact number is undefined . This number of people or things is imprecise; it is surely greater than two but not large. "the" is the definite article, which refers to people or things one has already mentioned or identified before. It could also refer to things in our general experience of the world or associated with everyday life. "The number of candidates waiting to be interviewed exceeded his expectations." Here it is a definite specified number of persons. "The number of "candidates could be for example defined as (20), yet he will interview "a number of "them that is not defined ,however it is greater than 2 and a relatively small number ,so they can't be for example 18 candidates . They wouldn't also exceed half the number. They will rather range from 3 persons to approximately less than half their number (10).