How high should you level Clarity as a mage?

I heard its best to not level clarity too much since the reserved mana will become overwhelming. Whats the best way to see how much reserved mana it will cost before you level the gem? And when should you stop leveling it as a witch?

There really is no clear answer to questions like this, since it all depends on what nodes you take and what gear you use.

It all depends on your mana pool. I've leveled mine past level 10 while taking equipment with mana regen to aid it. Some people keep it at level 3 for whatever reason.

Level it up and get mana regeneration passives and gear.

Super low mana pool is totally fine if your mana regeneration is high enough, especially since this means you can get rid of mana flasks entirely.

I have a lvl 75 shadow with only like 105 mana pool after mana reservation, but I have like 130 mana regen per second so it doesn't matter.

Personally, I think more regen is always better than a bigger mana pool, as long as you have enough mana to use your most expensive skill. A big mana pool that regenerates slowly is pretty much useless for sustained damage output.