I wonder how much percentage of all buildings are empty in China. In my visit I saw plentiful empty skyscrapers, especially next to decaying homes where people would still live.
Local governments have ways of telling, e.g. By electricity usage. You can also try counting the lights on at night to get an idea of apartment occupancy (a fun last time at the apartment complex I used to live in). Someone definitely knows, but you can be damned sure that this information is considered "state secrets."or