Skill tokens are special rewards for students who prove themselves particularly talented... either by mastering one specific Potion, or for being able to brew a lot of different ones!
For each kind of potion, when you complete your 3rd Potion of that kind (i.e., with the same power), you automatically receive a Skill token (but you don't gain another token if you complete 6 Potions of the same kind).
Also, when you complete your 5th different kind of Potion (i.e., completing Potions with 5 different powers), you automatically receive a Skill token (but you don't gain another Skill token if you complete a second set of 5 different Potions).
Skill tokens are received from the “countdown” stack (or from the general pool if the “countdown” stack is empty). Each Skill token is worth 4 points at the end of the game.
I should not have to point this out, but... better safe than sorry: you don't have to "create sets of Potion tiles" and "spend" them to receive Skill tokens. Think of each Skill token as an "achievement": once you have obtained it once, you can't obtain it a second time for the same reason. Also, a single Potion tile can be used to obtain more than one "achievement".
We suggest these numbers of Skill tokens so that your games last a reasonable amount of time (not too short, not too long). But, if you have particularly slow or overthinking players at the table, or if you feel confident enough with the basics of the game, feel free to agree on a different number at the beginning of play, if you‘d like to have shorter or longer games!
2 players: 4 Skill tokens
3 players: 5 Skill tokens
4 players: 6 Skill tokens