Interesting, this could be one of the first tests to see how "partisan" the Supreme Court is with the additions of Gorsuch and Kavanaugh.
The "Enumeration clause" is found in Article I, 1, § 2, cl. 3 of Constitution. After taking into account the removal and additions that have occurred with later amendments, it reads as follows: "Representatives ... shall be apportioned among the several States ... according to their respective Numbers ... . The actual Enumeration shall be made within three Years after the first Meeting of the Congress of the United States, and within every subsequent Term of ten Years, in such Manner as they shall by Law direct." The Enumeration Clause has to be read in conjunction with Section 2 of the Fourteenth Amendment, which states that "Representatives shall be apportioned among the several States according to their respective numbers, counting the whole number of persons in each State, excluding Indians not taxed."
The Constitution uses the word "numbers" or "persons" -- not "citizens," or "legal residents," or "those lawfully present", which is the argument that the lower courts relied on in rejecting the addition of the citizenship question to the upcoming Census. Complicating matters is the fact that the Constitution wholly and explicitly empowers Congress to sort out the details of the Census.
Notably, the strict constructionists on the Court will have to ignore the plain text of the Constitution and cite enabling legislation if they want to uphold the addition of the citizenship question to the Census, in effect accepting an argument that the name of first census act used the word "inhabitant" and that the contemporaneous definition of that word were persons entitled to the privileges conferred by the state, which would exclude unlawful residents. The word "inhabitant," is not used in the Constitution's Enumeration clause, but is instead used when describing qualifications of Representatives and Senators (the "Qualification Clause"). To get to the result that the Administration wants, the Court will have to read the Enumeration Clause as inclusive of the definitions of the Qualification Clause.
What this effectively means is that the roles of the Court will be most likely reversed here, with the strict constructionists needing to adopt a more expansive reading of the Enumeration Clause to add the question and the loose constructionists demanding a narrower reading of the Enumeration Clause to eliminate it.