Well, it's not a simple question. Article One speaks of the enumeration of the number of "free persons" - not citizens - for the various purposes stated. So, presumably, the census should count all "free persons" and not just citizens and legal residents.
It goes without saying that in the Age of Trump, there would be people in the country illegally, who, looking at the survey and seeing a Citizenship question, will either decline to fill it out or limit their response to those in that household who are in the country legally, if any.
Presumably, in California and other states with large populations of people who are not in the country legally, the enumeration will fall short of the actual number. Those affected states complain that, since the USSC has decreed - insanely, in my opinion - that those states must feed, house, clothe, medicate, and educate the children of those in the country illegally, the same as if they were fine upstanding citizens, they will lose out on not only Congressional representation (each H.R. seat being allocated to populations of 750,000 or so) but also Federal largesse that is passed out on the basis of population.
So we have the bizarre situation where these states generally identify themselves as "sanctuary" jurisdictions, where they decline to help the Feds enforce our existing immigration laws, and hence ENCOURAGE illegal intrusions into our country, and now want to insist that all of their illegals be COUNTED when they stand at the capacious teats of the Federal Government. [In California, anyway, where many of them VOTE and some pay taxes, I suppose this makes some perverse sense].
I would not BET on the USSC siding with the Government here. They may find that the citizenship question will impede the purpose of the census.
In my own silly opinion, I want the Gub'mint to FINALLY know and PUBLISH the actual number of illegals, so that the Media and the Leftists will get off this bullshit ELEVEN MILLION number that they have been claiming for more than a decade, and we can recognize the REAL number which is AT LEAST TWENTY MILLION.