There have been many, many more cases than the official confirmed (=medically tested/diagnosed positive) cases. For example data in Spain points to 5% prevalence (~2M infected) with ~25k deaths (official number, may be higher due to unverified and side cases).
Do you know if the serological data in Spain has also per-city estimates? I doubt the 5% is evenly distributed across the population, I'm expecting higher percentages in Madrid, for example.
If I remember correctly the data from Spain is also available itemized by region (autonomous community), and indeed the most population-dense regions have a higher prevalence. I believe the data isn't broken down by city, but it should be obvious that they're going to have a higher prevalence as well.
