number of cells in a city depends on the coverage you want to provide in your city.
If you say “the lowest rxlev acceptable in the city, outdoor, is -80dBm” -> that means you need x BTS.
Now if you want to increase this “design Rxlev” to -75dBm, you will need more BTS !
Each BTS (=3 cells) can only provide a certain amount of coverage. That depends on:
1- antenna height
2- antenna type
4- TX Power of TRX
5- TX Attenuation due to combiners and feeders
6- usage of Rx/Tx diversity or not
7- usage of frequency hopping
8- radio propagation (high buildings, lots of trees, etc -> that will block your signal)