According to Bulbapedia, Nimbasa has a population of 150, or 239 counting all the trainers in the stadiums. Castelia City has 127.
Note that's only the characters that are actually visible in the game. In reality you can't go into every single room in every single building in the cities. Given that Castelia has many skyscrapers the real population would be much higher. Also the people in the Nimbasa stadiums would likely live somewhere else and travel there to watch games.