Upvote:3
While the commonest Spaniard will have to do with just a name (that can be a complex compound one) followed by a paternal surname and a maternal surname (nowadays the order can be changed), aristocrats tend to spouse extremely long multiple first names followed by a plethora of surnames and family names.
For example, the current duchess of Alba is Eugenia Brianda Timotea Cecilia (4 names) Martínez de Irujo (first, very aristocratic, surname) y Fitz-James Stuart (second, and even posher, surname), daughter of María del Rosario Cayetana Fitz-James Stuart y Silva, the former duchess. She is normally known as Eugenia Martínez de Irujo, while her mother was universally called Cayetana de Alba (first name + title). Similarly, one of the nephews of the current king of Spain, Philip VI, is called Felipe Juan Froilán de Todos los Santos de Marichalar y Borbón although he is normally just called Froilán, given that there must be less than 10 people in the whole country with that uncommon name.
For the XVII/XVIII centuries the usual custom would be referring to the current owner of the title by a combo of name + title, where the name would be the one the person liked the most or that his/her family used the most, while the other relatives would be referred as name + surname, where the surname used would usually correspond to the branch of the family ostenting most important title, whether it comes from the paternal or maternal branch (a duke takes precedence over a count or marquis).
Upvote:5
A good primary source for the honorifics and the forms of adress of the Spanish nobility from the Siglo de Oro (the Golden Century, the period you are asking for) is the several plays by Calderón de la Barca and Lope de Vega.
From them, the rules are more or less: