Spanish plural last name noun phrases appear with a plural determiner and a singular or plural noun. Last names marked as singular are interpreted as a group, whereas plural ones are interpreted as collection of individuals (additive reading), although both behave like fully plural DPs. Based on a comparison with first names, I propose that last names involve a null nominal ‘group’ head dominated by an associative plural realized as the plural morpheme on the determiner. When the last name is plural, it has an additional NUM head that forces the additive reading. The last name number patterns are shown to be similar to those of N-N compounds, with which they share other similarities. The paper also surveys crosslinguistic pluralization patterns, proposing potential patterns of variation: the presence/absence of the null nominal and the structural location of the associative plural head.