At the current situation, Nether Portals are restricted to only be acessed on the Homeblock Chunk, meaning they can be placed anywhere else but the teleportation will only occur on those inside the mentioned chunk.
It's also denoted from my previous statement that there is no restriction of the ammount of Nether Portal a town may have, whether they are funcional or not.
In my opinion, this restriction is futile as it automaticily restricts the Nether Portal posicion as well as the town design, leading most of town to make underground Nether Portal rooms to keep their aesthetics.
Any and all portals you use in(side) the Nether will teleport to your homeblock upon exit. However, the only portal (or portals if we have multiple in the same chunk) that works to go from the Overworld to the Nether is the one that is located inside the designated Homechunk.
The 2 main reasons I believe this restriction was implemented was due to people using Nether Portals has a way to avoid/flee confront and to prevent massive appearences in the Wilderness of both "entrace" and "exit" portals. Therefore, the portals are all already "linked".