Charms are a special type of Item in Hollow Knight that provide various bonuses and special abilities. Charms use Notches when equipped and can only be equipped and unequipped while sitting on a Bench.

There are a total of 45 Charms, though only a max of 40 can be kept at one time, as 5 Charms can be replaced with different ones throughout the game:

Kingsoul for Void Heart, Fragile Greed for Unbreakable Greed, Fragile Heart for Unbreakable Heart, Fragile Strength for Unbreakable Strength, Grimmchild for Carefree Melody.

More facts here: https://hollowknight.fandom.com/wiki/Category:Charms