Fatigue
These dice are placed in your adventurer'sAdventurerEach character created by a player is an Adventurer. Each adventurer is a party member. cooldown trackCooldown TrackDice are placed here as the adventurer exhausts their skill dice or gains fatigue and status effect dice. The Cooldown stat determines the number of dice that are removed from the track each turn. when they gain fatigue. There are 2 types of fatigue. If a game effect refers to fatigue without specifying its type, that effect can apply to either type.
Light Fatigue
These are silver. When a light fatigue die is removed from the cooldown trackCooldown TrackDice are placed here as the adventurer exhausts their skill dice or gains fatigue and status effect dice. The Cooldown stat determines the number of dice that are removed from the track each turn., that die is returned to the supply. If there are no silver dice in the supply when light fatigue is gained, gain overfatigue.
Overfatigue
These are black. When an overfatigue die would be removed from the cooldown trackCooldown TrackDice are placed here as the adventurer exhausts their skill dice or gains fatigue and status effect dice. The Cooldown stat determines the number of dice that are removed from the track each turn. during the Recovery step, it is instead placed in the track's leftmost open position. Overfatigue in the cooldown track deals damage to the adventurer during fatigue rounds in battle. It can only be removed during the Adventurers Rest step, by taking an inn townTownThese encounters are described in the gazetteer and are triggered when the party ends its overland movement in a town or rolls a 
result on the unstable die while at an unstable landmark (see Traveling Caravans). action, or by a game effect or special ability.
Running out of overfatigue dice
v1.1 rule: If no black overfatigue dice remain in the supply, adventurers must take 1 true damage for each overfatigue they are unable to place in their cooldown track.
v1.0 rule: If no black overfatigue dice remain in the supply, no fatigue dice are placed.
SourceWhen you can take "any amount" of fatigue, is there a limit to this?
In a situation where you can take "any amount" of fatigue, there is no limit to how much fatigue you can take. The rules tell you what to do if you can't take a light fatigue or overfatigue, and also what to do if you'd take fatigue when your cooldown is full. Outside of those consequences, there is no limit.
This is particularly of note for the Palefrost Elk petPetPets protect adventurers from a number of harmful effects in Tamriel. However, each time the adventurer resolves a Pet ability, they must add an HP chip on top of the Pet card. A Pet with HP chips on it must be discarded after its HP limit is reached or the session ends. item which, when you would take any amount of fatigue, allows you to ignore the fatigue and put an HP chip on the pet instead. In combination with a situation where you can take any amount of fatigue, you can declare you are taking as much fatigue as you would like, and then ignore all of that fatigue.
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