The mana system in this game works different than the previous Dragon Age titles. In DAI, spellcasting essentially boils down to "How many spells can I cast with 100 mana ?"
The mana pool for all mages in DAI is fixed at 100. You cannot affect this. As such, what you have to do is engage in a bit of math to get the most out of it.
If you have noticed, almost all the spells in DAI have a fixed mana cost in that are multiples of 5. All of them. Energy Barrage, Barrier, Chain Lightning, Spirit Mark and Stonefist cost 50 mana. Static Cage, Walking Bomb, Pull of the Abyss cost 65 mana. Spirit Blade costs 10 mana. Fade Cloak costs 20 mana. Fade Step costs 0. All fire spells with the exception of Flashfire cost 35 mana.
So, time for a bit of math.
100 = 50 + 50 + 0 | (Barrier + Energy Barrage + Fade Step / Barrier + Chain Lightning+ Fade Step / Dispel + Energy Barrage+ Fade Step / Spirit Mark + Energy Barrage+ Fade Step / Horror + Chain Lightning+ Fade Step)
100 = 65 + 35 + 0 | (Walking Bomb + Fire Mine + Fade Step / Walking Bomb + Wall of Fire + Fade Step / Pull of the Abyss + Fire Mine + Fade Step)
100 = 35 + 35 + 0 + 20 + 10 (Immolate + Fire Mine + Fade Step + Fade Cloak + Spirit Blade)
The list goes on and on. My point is that to make the most out of the mana pool you have, be strategic about spell spamming. Use spells that when you use in succession, either has a total mana cost of less than or equal to 100. If not, you have to do a bit of autoattack which can reduce your overall DPS.
This is why the best spell in the game is upgraded Fade Step. It has 0 mana cost, is an Area of Effect and does 300 frost damage)
Here is the general formula
Assume mana regeneration = 0. Assume no passives. Therefore, effective spell casting is defined by the formula: S1 + S2 + S3 + ... + Sn ≤ 100 ; where Sn refers to the spell number.