It depends on the density of the towers, which depends on the density of customers and the availability of tower locations and the terrain. And the variations can be quite wide. In dense, urban areas, typically a single site actually has several antennas covering different "sectors." Imagine overlaying a compass rose on a map centered on the site. They might allocate 30 degree chunks to different cells, with each having a directional antenna pointed in the given direction. This is why the tower antennas often look like PA speakers - they're directional flat panel antennas.