Lawfag here.
Article I, Section 2, Clause 3 of the US Constitution states (actual language):
"Representatives and direct Taxes shall be apportioned among the several States which may be included within this Union, according to their respective Numbers, which shall be determined by adding to the whole Number of free Persons, including those bound to Service for a Term of Years, and excluding Indians not taxed, three fifths of all other Persons. (emphasis added) 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."
Why emphasis on the first sentence? Because it was later altered by the 14th Amendment of the US Constitution, Section 2 of which states:
"2. 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. But when the right to vote at any election for the choice of electors for President and Vice-President of the United States, Representatives in Congress, the Executive and Judicial officers of a State, or the members of the Legislature thereof, is denied to any of the male inhabitants of such State, being twenty-one years of age, and citizens of the United States, or in any way abridged, except for participation in rebellion, or other crime, the basis of representation therein shall be reduced in the proportion which the number of such male citizens shall bear to the whole number of male citizens twenty-one years of age in such State."
The first, second and third sections of the 14th Amendment all refer or relate to citizenship rights of existing persons, and those made citizens (y'all remember that thang called The War Between The States, right? That's where this Amendment comes from.)
Giving the natural, plain meaning of the original language of the Constitution is the cornerstone of interpretation of the law, and the term "free Persons" in Art. I, Sec. 2, Clause 3 can, in context with the remainder of that clause, refers to people (citizens and otherwise) who were lawfully present in the the United States at the time of the Census. People not of the age of majority (votors) and people who are not citizens but lawful resident alients, also have an interest in at least some of the rights and obligations of persons living in the US to the extent that a representative should be selected to serve their interest, even if they cannot lawfully vote for that representative.
However, in light of Section 2 and its plain meaning, when taken into context of the entirety of that Amendment which is not merely a statutory law but an actual, fully enacted Amendment to the US Constitution, which is the highest enactment of law in the land, it is made clear that the reference to persons who should be counted for apportionment of the Representatives in the United States is, in fact, limited to the count of citizens.
This in no way restricts the counting of the number of people existing in the United States, but it would apportion the Representatives according to the number of people whose full interest of rights arise out of their US citizenship.
And it is twisted logic to assume that people with fewer rights, due to lack of protection, would be denied whatever representative voice they are due because the apportionment of representatives was made according to the people who can (whether present or future) actually vote for those representatives.
It would be essentially the same thing as a guest in your home having a say in which bedroom you get to sleep in, merely because they are present in your house.