If you ask for a "table for five" or a question of similar nature where the number or adjective implies a noun following it such as "table for five (people)," should the number or adjective be in the nominative case or the accusative case (пять or пяти)?
Genitive: "стол для пяти (человек/тарелок)".
There is a special form for numerals which is commonly used in Russian in such cases: стол для пятерых. This is called "собирательное числительное" (collective numeral):
двое трое четверо пятеро шестеро etc.
They are a bit tricky: they can also be used with a noun (для пятерых человек), but only with an animate one and never feminine (для двоих братьев, но: для двух сестёр, для шести коробок). (There are exceptions for plural-only words). And, which makes it more relevant to your example, they more readily imply 'persons' as the noun, so "стол для пятерых" is a bit clearer and less potentially ambiguous (and more common) than "стол для пяти".
When used in the 'nominative' form (двое братьев), they formally require Accusative for the noun, but remember it's animate plural only, which is in practice often identical to Genitive.
However, in your example, the whole phrase (including the numeral) takes whatever form is required by the case: для + Genitive. We can put it in any other case:
Думаю о пяти/пятерых братьях (Prepositional) Горжусь тремя/троими братьями (Instrumental) Стол для семи/семерых братьев (Genitive) Стол на шесть/шестерых братьев (Accusative)