The part of speech that follows "millions" is typically a noun. This is because "millions" is a number that is used to quantify or indicate a large quantity of something, and it is followed by a noun to specify what is being quantified. For example, "millions of people" or "millions of dollars." This pattern is consistent with the use of numbers in English, where they are often followed by nouns to denote the object or entity being counted or measured.

Data from the Corpus of Contemporary American English (COCA) supports this observation. In the COCA, the most frequent part of speech that follows "millions" is a noun. The COCA is a large, annotated corpus of American English, and its data is reliable for analyzing language usage. A search in the COCA reveals that the most common nouns following "millions" include "people," "dollars," "jobs," "cars," and "houses." This indicates that "millions" is used to express large quantities of tangible and intangible entities, and it is always followed by a noun to clarify the type of entity being referred to.