TU is only the SINGULAR YOU.
VOI is only the PLURAL YOU.
DUMNEAVOASTRĂ is the POLITE YOU, used for both singular and plural. Even when used to address to a single person, the verb has to be in its plural form!
The verb is always conjugated according to the subject. So AVEȚI is plural because VOI is plural.
DUMNEATA (N., A.*) / DUMITALE (D., G.) is also a POLITE YOU, used only in the singular. The plural is "dumneavoastră" from the above comment, which can also be used for a single person. "Dumneavoastră" is more polite than "dumneata".
There are also third person POLITE pronouns:
DUMNEALUI (he), with the plural DUMNEALOR (they) --- N., A., D., G.
DUMNEAEI (she), with the plural DUMNEALOR (they) --- N., A., D., G.
DUMNEASA (he/she, N., A.) / DUMISALE (he/she, D., G.) --- rarely used these days
There are other personal pronouns, sometimes wrongly considered polite pronouns (in the region of Moldova they are used even for animals):
dânsul / dânsului (he, N., A. / D., G.)
dânsa / dânsei (she, N., A. / D., G.)
dânșii / dânșilor (they - all masculine or masculine + feminine, N., A. / D., G.)
dânsele / dânselor (they - all feminine, N., A. / D., G.)
*N. - nominative; A. - accusative; D. - dative; G - genitive
No. The important thing is the verb conjugation. "Aveți" - second person plural form of "A avea"
A avea - present tense
Eu am - I have
Tu ai - You have
El/Ea are - He/She has
Noi avem / We have
Voi aveți / You have
Ei/Ele au - They have
One can have more than one boy. Tu ai băieți?