We have a BTR-300 and last year one broken belt pack, the problem we ran into when we were going to replace the pack is the number of different versions there are of the btr-300. Each one has a corresponding set of belt-packs that match it. B&H photo video has 18 different versions of the BTR-300 for sale. Finding the corresponding belt packs for your station my be a fun task. We never ended up buying a replacement.
The
portable buttons turn on and off the communication with the belt pack associated with that number. The BTR-300 packs are fixed Crystal meaning they can not change frequency. So once you buy the one that goes with
portable switch 3 it will always be associated with
portable switch 3.
That aside, in theory you could have more than 4 packs listening because they would not interfere with each other when just receiving. Each BTR-300 Belt Pack has a common frequency that all of the packs for that particular
base station listen on and an individual frequency for that particular pack to talk to the
base station. People with the packs that are doubled or tripled up on the same frequency would have to be very carful about turning there mic off after they finish talking. This may prove difficult because it easy to leave your mic on or forget about it. After a mic is turned off there is also no beep or tone to signify that the air is
clear and it is safe for everybody on that particular frequency to talk. When they they are not trying to talk over each other on the same frequency they would be able to talk in
full duplex.
I hope i was able to answer some or all of your questions.