The maximum range of a cell tower is only useful if you are in the middle of nowhere where you don't expect many people to be using their phones. The real limitation is that a tower can only support a limited number of simultaneous connections. In order to solve this problem, the carriers adjust the radius of the tower, by adjusting the downward tilt of the antennas and probably the transmitting power as well. In very high density areas the radius will be very small, so that they can install lots of towers to support the large number of users. This means that calculating how many towers you need is not a simple mathematical problem, but has to factor in population density, subscriber density, and knowledge of which areas are currently experiencing problems--as I am sure that are lots of areas where data speeds are just fine.