In the recent interview Koji Fox said Light and Dark are not elements just aspects, so you could, theoretically, have 4 different Primals all with a lightning element but one is Astral Light, another Astral Dark, then Umbral Light, and finally Umbral Dark. I am basing this on what he said though honestly it didn't really seem to answer what Astral, Umbral, Light, and Dark are just that they are not Elements. But as far as the Primals we've seen thus far are the original 6 all had a single Element
Ifrit: Fire
Titan: Earth
Garuda: Wind
Leviathan: Water
Ramuh: Lightning
Shiva: Ice
and Phoenix : Fire (I swear everyone forget him, lol)
The Warring Triad Each have 2
Sephirot: Earth and Water (his attacks and such while not a direct call besides Earth Shaker and Storm of Words, all draw inspiration from Kabbalah, connecting to the Word Tree and Knowledge. Trees are a part of nature which is either the primary or secondary aspect of the element of Earth and Water is usually a representative form of Knowledge, think pitchers and the Ewer card. It is also worth noting his original element from VI was actually Blizzard making him the only Triad member not to keep his original element)
Sophia: Lightning and Wind (While these reasons may be unintentional, they still fit her aspect. Sophia's original element from VI was Lightning and that stayed. Thematically Wind is the exact same as Lightning just not ionized, Lightning and Wind are sometimes combined as one element in some elemental depictions. Also Sophia theme is balance and in order to create Lightning and Wind a certain balance of atmospheric pressure is required.)
Zurvan: Fire and Ice (Zurvan's original element from VI was Fire, and he appears to have gained Sephirot's Blizzard aspect. Zurvan's title in Japanese is Kishin, and a Kishin isn't a good or evil god per se but more like a forceful or fierce god who loves battle and often causes destruction; basically gods that govern aspects that we don't view as beneficial but are a part of the world, like a god of death (Thal) or god of war (Halone). His aspects of fire and ice reflect this somewhat. Not only are they opposites, but the main perception for both is the opposite, Fire is inherently bad and Ice inherently good; but each is just as beneficial or as deadly as the other if generally perceived.{The other 4 elements are a bit of a mixed bag. While they could just as easily be good or bad we generally approach them with both aspects in mind from the start. But as animals, our base instinct is fire is dangerous and Ice isn't, but we as humans with our higher intellect are able to see the benefits and dangers each possesses})
and the initial HW Primals have 3
Ravana: Earth, Fire, & Ice (For what reason these elements were chosen for Ravana is unknown. Possible reasoning could be that: his beast tribe is bug inspired and bugs usually live within the earth, this depiction is very war-like and more war-like "gods" are often depicted having an association with fire, ie Ares. Finally, in mythology Ravana is a devotee of Shiva. Since in the FF universe Shiva is an ice elemental, him having a minor aspect to it might hint at that. This also makes that initial fight kinda funny, lol)
Bismarck: Water, Wind, & Lightning (Bismarck's reasoning for the elements he has seem simpler, they are all aspects of weather, which Bismarck in XIV is said to control. Also there are the facts that; Water is because he is a whale as well as his name being based on a ship [based on a German man]. Wind because he is now a "Sky"-Whale, and Lightning, since it the most common depiction of weather after rain.)
Those are the ones that can be considered definitive. The remained are a little bit more up for interpretation. While it may be hard to find definitive elements for these Primals, figuring out their aspects might be easier
Odin and Alexander appear to be complete opposites as they were in XI. Its easy to guess that Odin is Dark aspected and Alexander Light aspected. But based on their lore and interpretations, Odin seems to be Astral(active) aligned and Alexander Umbral(passive) aligned
As someone earlier said Bahamut is stated to be Astral aligned, and they said that Shinryu is probably Umbral. This seems to make sense given how each was summoned. And as for the two kings, Mog is probably Astral, based on his lore, and Thordan Umbral, based on how he went about doing things.
As for the Two newest Primals, here is what I think is a good interpritation.
Susano: Water and Lightning, Astral & Dark (His Element of Water makes sense considering his beast tribe is water based. Plus his element of Thunder makes sense given his basis is the god of storms in Japanese. The Astral aspect is probably referenced in his actor personality as well as his summoning. Koji Fox said that his summoning is exactly the same as the other primals, and while it might not seem so the Kojin, "actively" worship the treasures used to summon him. His Dark aspect I'm basing mostly on his color pallet but Dark is not always evil.
Lakshmi: Earth and Fire, Umbral & Light (Lakshmi's elements might be more of a stretch since she has no attacks indicating as such, but I'm basing it on both her beast tribe and inspiration. She and the Ananta are snakes a terrestrial creature, they also primarily deal in gemcraft. Gems come from the earth, and fire is use to mold and shape the final product. Lakshmi is also based on the Hindu Goddess of Wealth, Fortune, and Prosperity. Earth and Fire are the prime elements to depict such things; Earth, land and money, and Fire, Passion and Drive. The Umbral aspect is mostly because of her summoning, she wasn't summoned conciously, and because of her overall attitude, "Ignorance is Bliss". The Light aspect is mostly because of her color pallet too, but Light is not always good.
Those are my interpretations anyway