Cel/cea/cei/cele is used as an alternative definite article when it is not possible to suffix the usual article.
That's because the article is normally suffixed onto the first element of the noun phrase: usually it will be the noun itself, but if there is an adjective comming before the noun (which is quite rare as most adjective come after the noun) then the article is put on the adjective:
bărbatul bun - the good man
primul bărbat - the first man
However, the definite article cannot be suffixed onto numerals like două, and numere is "too far back" to accept a suffixed article, so the only way around is to use a demonstrative adjective as a replacement, which is the role here of cele.