How far a scanner can receive depends on many factors including but not limiting to what you are monitoring. As for your second question, yes and no. Many law enforcement agencys are encrypted and others are not and have no plans to do so at least on the main channels.
Absolutely! A good example might be the transmitter antenna height. We usually think of building roofs, or mountain peaks. But remember, there are organized groups that listen to orbiting satellites, some of which are many hundreds of miles away. And if your going for distance, the moon would be a good place to install your transmitting antenna, lol.
On my RS Pro-106, with no external antenna, I’m picking up most everything, police, fire and ems (conventional & trunked) in the 15-20 mile range. Located in Cuyahoga Falls, Oh.
The distance a scanner can receive a signal depends on how strong the signal is, what kind of antenna you are using and what frequency you are trying to listen to.
What frequency range are you trying to hear and what kind of antenna do you have?
Really too many variables to give an accurate answer, but many years ago when I was living in southeast Alabama I did, on occasion, pick up the California Highway Patrol on some of the Low-Band frequencies they were using at the time.
Frequencies above 30MHz are essentially line-of-sight. That is they don't travel beyond the horizon. There is some refraction and things such as E-skip that can make them travel further but basically once it hits the horizon it stops. The higher the antenna between the transmitter and receiver the greater the distance that can be achieved due to the curvature of the Earth. Once you get into 'ground clutter' it gets very convoluted. Generally higher frequencies are more direct than lower but tend to penetrate things better. So you may get 45MHz from quite some distance as it bounces around but only with an outdoor antenna. 800MHz only goes where intended but can be heard inside a concrete building. These are broad generalizations.
Also there is a matter of signal strength. A good example is fire ground. These low power mobile and hand held units are for fire and rescue workers to communicate on a VERY local basis. They can be just a few blocks away and a table-top scanner with built-on antenna cannot hear them. Put a receive antenna up on a tower and you're above the trees and buildings and may be able to hear them well.