Your maximum Mana rises only when you level. Keep your Perception and Intelligence high and you get more Mana per level. There are also some items that will give you a few more Mana points when they are equipped.
It's not hard to imagine that a Mage could not take on a whole army at once. Being able to kill nearly two Taurex with your Mana reserves is pretty good actually. Beyond that, for large fights, you are going to need some Mana Potions to boost yourself, just like a Fighter would probably need Health Potions to make it through a large battle.
If you don't want to rely on potions, then learn how to separate enemies from each other so that you don't have to take on large groups at a time. Isolate and destroy them.
I just finished the game as a hybrid mage with some rogue skills. The issue is not taking on a whole army at once - the issues is that the mage has to rest after every single fight
. Every one. And it gets worse as you move up in level, because while spell damage goes up, so does the spell cost and the amount of damage you need to kill the mobs. Essentially, the percentage of your mana that you spend on the spell stays roughly the same. I finished with 149 mana, which meant I could cast Deep Freeze 6 a whopping four times. Four. I *might* kill one Taurex, if he doesn't resist.
And actually, recommending separation of enemies is good until you get AoE spells - then you want to herd them together, since your AoE spells are far and away the most efficient method of "shoot shoot rest". Hitting two mobs with Compress Atmo is a huge damage/mana improvement over any single-target spell - not to mention that Compress Atmo gets resisted a LOT less.
This was my climactic attack on the Goblin Citadel with my Giant allies:
Enter the Citadel gate. Cast Air Shield. Aggro the initial goblin group. Run away to herd the goblins together. Kill three goblins with two Compress Atmo blasts, and damage a fourth. Kill the fourth with a Fire Dart. Now I have two goblins left, and a goblin warlord. But ... I'm out of mana. A mana II potion will give me one
more shot with either Compress Atmo or Deep Freeze, which isn't enough. I'm not swimming in potions, so I run. Run run run, getting back 1 mana every other turn or so. Turn on them and hit them with Compress Atmo twice. The two lesser goblins are down, but the warlord is still coming. Run. More running. I could take a potion, but why bother when it'll only give me one Deep Freeze shot? Save it for when I really need it, like when I'm stuck in a room and can't run. So I run. Run, run, run, run. Finally, turn and kill the goblin warlord. Now I'm halfway across the zone.
Rest for ten seconds, get all my mana back. Run back to the gate. Run, run, run. Giants are here now, getting their butts kicked because I'm not in the fight. Kill one goblin with Fire Dart. Start on another one. Uh oh, I've attracted the attention of a Taurex. I'm at half mana, which isn't enough to kill him - it's not even enough to fire two Deep Freeze 6's. I Fire Dart him a couple times, both resisted. Wonderful. Run, run, run, out into the desert with the Taurex on my heels. Round and round the campfire we go. Deep Freeze. Run round and round and round. Fire Dart. Resisted. Is this guy made of fire-resistant material or something? Run away again. Fire Dart. Hits for wimpy damage. Run and run and run. Run more, we're going to Deep Freeze him this time. Deep Freeze. HIT! But ... didn't kill him. Run away again. Run run run. Fire Dart. FINALLY dead.
Rest for ten seconds, get all my mana back, pray I don't get attacked. Run back to the Citadel gate ... and the fight's over. One giant still up. Still a few goblins in the corners. I clean them up one or two at a time, resting for ten seconds between every single fight.
Essentially, I missed most of the fight because I can't afford to quaff potions like Tylenol. In game time terms, I spent 99% of the coolest battle in the game resting to regain mana
. I probably would have needed 20 or 30 Mana II potions or 10 Mana III potions to participate in the Citadel gate attack with any sort of staying power. I spent almost all of the fight running for my life, trying to build mana in order to shoot.
It's an eminently frustrating mechanic.