Here "Die Ratgeber" would also be correct, as "Der" is the article for singular form, and "Die" is the article for the plural form, which is the same in both cases.
While your pondering about the German numerus is correct, it would be 'the advisers' in plural.
I disagree. If you are given the English and asked to translate to German, "Die Ratgeber" is wrong, as the English is singular (the adviser, and not the advisers).
Also, hearing 'Ratgeber' makes me rather think of a guide(book) than an adviser. Adviser is more likely 'Berater'.