Why do cell phone companies need so many base stations?

If there were enough frequencies for each cell phone to have its own frequency, only a few, centrally located base stations would be required.  However, since there are only a limited number of frequencies available, and since there could be tens of thousands of cell phones in an urban area, the same frequencies must be used over and over again.  This is accomplished by establishing small areas, called cells, that have a small enough number of callers on the system so that each can have one of the available frequencies.  If too many callers want to use their phones within one cell, they will get busy signals, and the carrier will get complaints, leading them to squeeze in an additional cell, making all the ones around it just a little smaller.