fluent2332: I just played and beat the game as a Mage. Never had a problem with mana potions, although yes, you will go through them like nobody's business. I'd run out of them occasionally and just visited a merchant and bought more. I also created a bunch of pure mana potions as well. Make sure to stock up on meadow knotweed and lab bottles when you can.
I actually messed up my character a lot, I think. Ended up with like 270 mana, which seemed low as I couldn't even use a Level 6 run like Shrink Monster. But at the end of the game I finished it with a couple dozen extra mana potions. There's tons of them and unique ways you can afford them, just have to get creative. Sell things, make gold and load up on them at a merchant when you can. They also respawn at merchants between chapters.
And yes, you have to manage your mana well. If you can kill a skeleton with Ice Lance instead of Lightning, do that. Don't waste higher level spells on weaker enemies, sleep in a bed when it's convenient and don't be afraid to summon creatures to help you. Also, scrolls give you the same spells at a highly discounted mana rate, so instead of Lightning costing 35 mana (or whatever) to cast, it will cost 5. There are tons of scrolls as well, so use them. It helps a lot.
Eventually, they can run out - merchants only stock so many. I learned the hard way that, when playing as a mage, do not try to wipe out the orcs or you WILL find that there are no more ways to both make and buy mana later. Now, I'm going out on a limb because it's been so long since I played and I have a terrible memory but, in at least one Gothic game (I,II,III), a mage can regenerate mana late game and I'm thinking it's II. Whichever one it was, being a mage became quite easy after that.