Thank you for the feedback! It's probably not very clear, but the potions are scattered throughout the level and you have to bring them to Merlin so he can drink them. The big Mana pool is your reserve and the small one is your immediate useable Mana.
After all the feedback and play testing, the other developer and I do agree that the Mana availability is way out of balance! We have an updated build that is much more Mana generous, and we'll probably release that after the jam.
Thanks again!