I think now, Sovereigns generate 2 mana per turn (or 4 with Attunement), Shards controlled generate 2 mana per turn (as of 1.09o), and Greater Mana Shrines generate 3 mana per turn.  And AFAIK, you can have a negative mana generation rate, where you will actually lose mana per turn.  And I would think, given that Heavenfall's Faction mod allows the Goblins to start with negative mana, you would go into negative mana eventually, which would have the effect of preventing any new spell-casting until you have at least a positive amount of a particular spell's cost.  Not sure if spells requiring maintenance would be automatically cancelled if you had negative mana (but my (complete guess) would be no).

Best regards,
Steven.

Summon familiar and arcane weapon are very cheap to cast, and for a reason. Cast them for 1 mana, and dispell them when you don't need them. Save your mana! There dosent seem to be too much of a limit to how much you can cast from the overland map, I quite often find my self casting  weapon on all units before  a battle then dispelling it after if their is no combat straight up.