"Señora Castro, ¿usted necesita sus vestidos ahora?"
Translation:Mrs. Castro, do you need your dresses now?
Yes, that is. Possessive adjectives have to agree in number (singular / plural) with the noun.
The subject (Mrs. Castro) has nothing to do with it. 'su' is used if the object referred to is singular. When the object is plural, like it is here, you use 'sus'.
I believe you are trying to match the verb to the gender and number of the object... which is incorrect. You need to conjugate the verb to the subject: 'necesita' is third person singular (used for él, ella, Usted, or in this case Señora Castro) while 'necesitas' is second person singular informal (used for tú).