By "channels" you mean talkgroups? As they said, you'll only hear ones near you that go thru that SITE. But there are some talkgroups you can hear from far away if they're statewide. For instance, I hear IEMA Lincoln IL Weather up here in the Chicago area any time there's bad weather. I heard recent flood reports from the IL Emergency Management talkgroups way down by Quincy IL and the St.Louis area. Same thing happened with IDOT "statewide" talkgroups when I heard repairs being made to ramps down in Springfield IL/Sangamon County. I make a point to include all Emergency State talkgroups or IDOT even if they seem to be out of my SITE tower range. You never know what you'll hear.for your state.
Just to sharpen the pencil a little. There are three different scenarios when it comes to trunked systems (and even some conventional ones) with multiple transmitter sites.
This is where there are several transmitters covering a given area, usually (but not always) for a large metropolitan area. All of the transmitters transmit on the same frequency at the same time. This is done for saturation purposes.
This is where there are several transmitters covering a given area, usually multiple counties. Each transmitter is transmitting at the same time, but on different frequencies. User radios determine which signal is best for them and lock in on that site.
OmniLinking (applicable to Indiana, usually):
There are also several transmitters covering the given area. Typically each transmitter has its own different set of frequencies. However in large statewide systems, frequencies could be used (South Bend tower may use same frequencies as Charlestown).
Radios typically affiliate with the site that gives them the best signal. A Sellersburg trooper in the Floyds Knobs area will affiliate with the Floyd's Knob site. At the same time, another Sellersburg trooper may be affiliating with the Henryville site because that gives the best signal. When that situation occurs, Sellersburg traffic is broadcast on both sites.