Identity (peronality,ego,..) of planets
tl;dr; Planets have self-identity, portals connects such identities, so their are unique for each pair, but there are more theory around and some special cases...
Each planet is unique so it is its self-identification (or ego, or personality or call it as your magic tradition is used to call it).
From magical point of view it is somehow based in the "inner heart of planet", which is usually somewhere near the centre of its mass.
And potencial portals between planets can be represented as distance between their hearts - which naturally give only one way for any pair of planets. (And any two planets can be connected this way.)
There is the problem, that each planet more or less consider itself as "center of universe" and feels other planets as far as their personalities differs. Which makes total mess of what is from magic point of view about "straight" "close or far" and "at which direction" - something like if you would like map relations in large group, whith respect to their affinity - just not on paper but in 3D.
Basically the planet "feels" its ouside as its more important part so potential for portal is placed usually at landside, or at see level, while some where found in mountain, even in caves and there are theories, that some may be underwatter (more often between sea worlds with fish-like inhabitants) or even underground inside rocks or lava (for planets, that hates each other?).
Usually planets with closer identities allow for larger, shorter and relatively cheaper portals.
Magican and senitive beings can feel, where the potential portal end is placed, say in similar way as for magnetism or electricity ("my hair ticks here and sometimes even rise and flows as in the wind"). A lot can be found from carefully observing such place. Still it takes a considerable effort to perfectly pinpoint the right place and then open actual portal there - which takes also some time (like days or even months or years) from the the point when entrances starts to manifestate on both sides to the point when portal connect those ends and is wide enought to be usable.
Some tests was done, but the place is 3D restricted, so there is given the size of the portal (usually omething between narrortunnel, which must be crawled thru, to really big gates, where even two warships could flow side by side) a well as its position (and magic of such force cannot be controlled so preciselly to prevent the portal entrancess to span over their full size before reaching the other planet.
Even trying to create two (or more) portals at the same time, from the same size or from opposite sides, resulted only to merge of magic sources and faster opening.
The same goes for eventual closing of portals, which shrinks "the tube" slowly.
Opening and closing works against each other, deducting the manas used from each other - so final effect takes longer, if one side can pump sufficiently larger quantity of mana into it.
Small quantity of mana does not affect portal at all, as it have some natual magic inertia, which dissipates some amounth of mana trying to affect it - so if there is not enough mana to open portal fast, it would not open at all and dissipate the invested mana to emptiness while it shrink to itelf old "potential" state. But once a "critical mass" is reached, it will open to its fullness and stay so nearly indefinitively, dissipating eventualy small amounts of closing mana.
There are some problemss, as the identity of planets changes over time (say from nearly empty world to overcrowded one, river and seas may dry out or change substantially, some major pecies can die-out or even big enough space body can hit the planet) - which affects the identity of planets as well as their relations.
Small changes, as wars and plague and such does not matters for planets sso much, so the change of portals is usually not even measurable, but bigger and long term changes may start to move portal places, so the portals may move too. As magic constructs, they just ignore terrain, buildings, restrictions and move thru, eventually diplace/tranport obstacles to their other end. If the change in planet identity so too big and fast (at planet terms, not human ones), then the portal cannot follow it and is destroyed, resulting in eventual disasster around old gates (a lot of magic released uncoordinated).
There is even way to create two portals between the same pair of planets, in the theoretical case, when one planet get "split personality", while it is not clear, what could have such dire result and not destroy all live on such planet, or at least half of it.
Also is theoretically posible for two large space bodies with high enought personality for portals (so probably small moon or larger) to colide in way, that they till keep it personalities, while effectively merged in one space body - like crash big moon to Saturn and let it survive as single body, mainly enclosed in other - such two-planet could have two sets of portal on one space body. (But usually one or both planets are so damaged, that their personalities are destroyed and eventually new personality is borned) - probably noone would survive such impact, but portals to such double-planet D1-D2 could be found from outside O and while extremly rare, another portal on the other part of D1-D2 may even be not so far from the first - which could shorten travel on planet O by shortcut O--D1-D2--O.
Still more probable shortcuts are A-B-C-A (or longer chains), where A-B and A-C portal are on A far away and A-B is near of B-C on B and B-C and C-A is near on C.
Also I would suggest, that transport is not entairly for free (to avoid tornados, flows etc.. from different conditions on different ends), but some "magic" had to be performed to allow somebody to enter the "tunnel" - and it would open one side of the "tunnel", let enter as many entities and cargo as could fit in, then close the gate and open the other, with slowly growing force to empty the "tunnel" before it closes after some time again.
Who was strong enough to stay inside, or was not able leave the tunnel, he was lost forever and never heard of him again - so portal ending undeground are really feared by explorers. (yes, execution by dead-end tunnel is posible. And sending slaves this way with tools to make a small chamber at the other end and then return (if they are able pay "magic toll") is possible too)
The "magic toll" is same for both direction and can range from hard rituall and plenty of magic for too different "peronalities" to simple spoken pray from mundane for really close peronalities (like "Oh mother earth, please let me step inside and travel safely" from atheist and not the same phrasing is required, just the intensity of will/mana)
Also while tunnel is open, anybody can entry - so there may be "porter", who will open it for you, but not entry at all and wait for other travelers. Some skilled magican may be able even formulate toll for "way back" which would open portal from other side.