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You'll want to open ports if you want to access CouchPotato, Plex, or Sonarr from any place other than your house (for instance, on your phone using the CouchPotato app, etc). Or, if you want to be able to stream your movies and tv shows from anywhere, you'll need to open the 32400 for Plex. All I did was go to this site, find my router, and follow the instructions therein. Depending on your router, it shouldn't get too complicated.
On the mapping of ports: you can map the ports to whatever numbers you want! I just used his suggested port numbers for ease of remembrance. But the default ports for the applications may be a bit easier for you. They go like this:
CouchPotato: port 8080
Sonarr: port 8989
Plex: port 32400
SABnzbd: 8080 (change this or CouchPotato's port to something else like 8081 to avoid conflicts)
You don't have to open ports, but I would recommend it. Do you know your router's manufacturer name and device model number?