If the gov’t can send me a Census form pages long, well beyond a simple “how many live here”, and send a Census agent to my home to “remind” me there were more questions for me to answer beyond a simple “how many live here” (yes, literally happened, he was parked there waiting for me to come home, I told him “I answered what the Constitution requires” and he got the point from my tone that leaving was a good idea), and has enacted law fining people $100 for not answering all questions, then heck yeah they can ask “how many people living here are citizens?”
If SCOTUS rules that asking “how many living here are citizens?” is NOT allowed, then that will cause precedence for eliminating all other questions beyond “how many live here?”.
Last census I only answered how many lived here. A census worker came to my door to ask me the rest of the questions.. She got sneaky using ‘small talk’ to get more info out of me. I said “Hey, you’re trying to trick me!” I must have scared her.. She quickly left. Heh.