Be sure to note, in addition to Kevin's answer, that you might want to test exactly your situation's ratio. If you are playing on PC or console, and if you are playing on console which one, and which world size. All of those things allow for radically different ratios. You can test it yourself in a creative world with the same map properties...just build a few dozen portals and experiment a little with them.
An additional thing to note is that portals will seek out the nearest available portal if you build them too close to each other, with exceptions. One with careful placement and activation orders can link portals in complex but specific ways.
PC example of the concepts:
In one of my worlds (Xbox 360) I have an underground pyramid. At the top of the pyramid is a portal which anyone will find on their way down from the overworld. If they enter this portal they go to the nether hub, if they enter the portal in the nether hub they return to this top portal.
I have this or a similar design (one of Mumbo's probably) nearby in my pyramid:
This hidden portal actually turns /off/ as you teleport. It takes you to a secret room BELOW the nether hub portal. Taking the portal out of there...well the portal you took to get in is hidden/turned off so it "doesn't exist" and the game will want to create a new exit portal for me, but I purposefully placed it close enough to the "normal" portal that instead of creating a new portal close to where the "hidden" one is it just sends you to the former.
Oh this isn't always clear but only the x and z, or the horizontal planes, matter. They can be any /height/ difference and still link up, but once you move them apart from each other horizontally in any direction that is what changes the linking.
Using these techniques one can create a very intricate hidden travel network that hides behind the "surface" travel network. Very fun stuff for being an admin in a game without giving myself admin powers...I just build the world itself to be my "admin powers". :)