You can buy a really expensive police scanner and listen to a channel at a time, or buy several expensive scanners and listen to a few. Then you'll still need some sort of way of recording them and well, it's all a mess.
With a powered USB hub, some cheap software defined radios (re-purposed european tv tuners really), antennas, and a raspberry PI (or other linux box) you can build a scanner that will receive, decode, and record every single conversation happening on every talk group that your radios are configured to recieve.
So yeah:
- Powered USB hub with enough space between ports and enough ports to fit all your SDR dongles.
- Enough SDR Dongles to cover the frequency plan for the trunk group you want to monitor.
- Optional shortish USB extension cables
- Maybe not optional RF bandpass modules (in case your local AM/FM signals are too hot and causing interference)