When creating more than one portal, they all link to only 1 portal in the nether. (This is a survival world. Do not have a creative Realm world to test in).

First was seen when creating two over world protals close but should have been far enough apart. However they still linked to the same portal in the Nether. Breaking the nether portal (either by removing obsidian or by breaking the nether field in the portal) will allow the second portal to create a new portal in the nether

Assuming that I wasn't far enough away, went for "days" in the over world and create a new base away from the spawn base camp. Creating a portal there again linked back to the start portal in the nether. Returning though the portal resulted back into the original spawn base camp. Breaking the nether portal in this case still created a new nether portal, but one quite a distance away as expected. Hopping through the portal in the base camp jumps to this new portal (not deactivating original portal or creating a new one) and going back through now goes to the new base.

Breaking the "new" nether portal, and jumping back in at the portal made at the new base, creates a new nether portal on top of the original in the nether, instead of reactivating the existing one.

TLRD - multiple over world portals link to a single nether portal, and the single nether portal links to just 1 overworked portal, creating basically one way trips.