It depends. If you open a portal in the overworld, there will be a portal leading back to the overworld, but it may not lead you to the same spot if it linked to an existing portal in the nether instead of creating a new one.
If I create portal A in the overworld, the game will automatically create portal B in the nether so that I can get back. If I create another portal, portal C, within 1024 blocks of portal A, it will link to portal B. So you can go A -> B, C -> B, and B -> A, but there is no way to get back to C from the nether other than to go through portal B and then walk from portal A to portal C.
If I create portal A in the overworld, the game will automatically create portal B in the nether so that I can get back. If I create another portal, portal C, within 1024 blocks of portal A, it will link to portal B. So you can go A -> B, C -> B, and B -> A, but there is no way to get back to C from the nether other than to go through portal B and then walk from portal A to portal C.