Doubts about plurals of first and second names in Spanish? Here's a quick reference, according to the Real Academia de la Lengua Española (Rae.es)
First names have their plural according to the general rules.
Some examples: Singular: María. Plural: Marías Singular: Javier. Plural: Javieres
Second names (Family names) keep the same (singular) when they refer to the members belonging to the same family: Los García son una familia agradable. Note than in English we add a final -s: The Smiths.
When second names (family names) refer to a group of people with same second name, but from different families, both forms, singular and plural can be used. The most common use is the singular - that is, to keep them invariable- mostly with second names that can also be used as first names, to distinguish one from the other. For example: "Los Benitos de mi clase son muy inteligentes." In this case, "Benito" would be a fist name. But: "Los Benito de mi clase son muy inteligentes." - Here, "Benito" is a second name.
Second names (family names) ending with -z will keep -always- the same form, singular. For example: "Los Fernández", "Los Gómez", etc. - They have no plural form. (We can't say: "Fernándezes" or "Fernándeces", that wouldn't be correct.)