Translation:Mrs. Castro, do you need your dresses now?
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).
In Spanish you have two kinds of 'you' for addressing one person (the plural 'you' is 'ustedes'), the informal 'tú' e.g. for friends and family, and the formal 'usted', for addressing anybody you are not acquainted with, or somebody with authority, e.g. the doctor, your manager, the person you ask for directions, etc.
'Tu' and 'tus' are the possessive pronouns linked to the informal 'tú'. 'Su' and 'sus' are the possessive pronouns linked to the formal 'usted'. There are two possessive pronouns for each because Spanish makes a distinction between possessing a singular item versus possessing multiple items:
tu camisa = your shirt (informal) tus camisas = your shirts (informal) su camisa = your shirt (formal) sus camisas = your shirts (formal)
To make things even a bit trickier, the possessive pronouns 'su' and 'sus' can also be linked to 'his' or 'her':
su camisa = his/her shirt sus camisas = his/her shirts
I am perplex. I am Francophone and get mixed up between Ma'am and Mrs. In another post smo explain that in english if you talk directly to smo your would say Ma'am and about smo you would say Mrs. Here Ma'am has been put wrong.
Seriously the pickiness of the translation to english feels way to narrow. When learning a new language, the difficulty is supposed to be translating TO the new language. I am over all pretty good in English. Nevertheless that is where I am wasting time....
I'm sorry someone in California got mad when you called them, "Ma'am"! I moved to California from the midwest 40 years ago, so I must say that some Westcoast women don't like it because they consider Ma'am an expression respectful to "old married" ladies. They would rather you call them "Miss" unless you know they're married. You'll get a positive response, trust me. I call female strangers "Miss" from teens to ladies in their 80s.