So I decided to review some guides just to learn more and I noticed that most guides for Ahri use scaling health over mana regain or armor. I tried using these runes and I just couldn't make them work. Could anyone explain why those runes are recommended?
Against an AP mid you don't need the armor and the health is useful for staying alive. Scaling health is more beneficial than flat health if you can survive well early game, which Ahri can, but around the mid game you should start to roam more, and you'll be getting more benefit out of scaling health in the midgame than you would out of flat hp runes. More HP also works fairly well with Ahri's passive, so you can heal up even more.
If you're against an AD mid, take armor seals pretty much every time. Mana regen seals are kind of a waste once you get morellos/chalice/blue buff so there's no point in running them. You do use a lot of mana early, but the key is to conserve mana until you get your mana regen item, so that means you're mostly going to be aa'ing to farm with an occasional Q to clear a wave (and hopefully harass your opponent at the same time). Don't be afraid to use your full combo if you're sure you can land the charm though, it's well worth pushing your opponent out of lane for a little while.
every rune page is situational. honestly, if you use health runes, its because you're looking to survive some form of burst from an enemy by living with extra health, it doesn't reduce any damage whatsoever, like armor or magic res, but it can help you live through damage. It's universally legit vs all kinds of match ups. Magic resistance is good for douple AP comps or if you just want to resist some magic damage in general. Mana regan is good if you think you may need some more mana, for laning(cs'ing vs a heavy lane clear opponent) or throughout the game. Armor runes is legit, period. Everything in the game hits for AD. Except those hybrid champions or AP. Which is why i prefer Armor runes in general, because you can dodge AP skillshots, granite they do true damage basically if they land their combo, but that's where the skill comes in, when you dodge it. But if they aren't a skill shot based champion i suggest health or MR.
