In Spain:
-¿De dónde eres (tú)? - Singular
-¿De dónde es (usted)? - Singular formal
-¿De dónde sois (vosotros)? - Plural
-¿De dónde son (ustedes)? - Plural formal
In some places in Spain (Islas Canarias) and South America there are variations. I think that in Argentina, for example, they would say "¿De dónde sos (vos)?" And in other countries they would prefer using "usted" instead of "tú", unlike in Spain. If I'm mistaken, please somebody correct me :)
Cheers.