Hollow Knight's 45 charms with their numbers, not prose. Hover one you don't wear and see what it would change before you equip it: nail damage, hits per second, how many hits you can take, soul per hit, healing time and every spell with its variants. Notches count as in the game, overcharming included, and synergies (Sharp Shadow with Dashmaster, Spore Shroom with Defender's Crest, Grubsong with Grubberfly's Elegy…) are detected on their own.
An example: Nightmare King Grimm has 1500 health. With the Pure Nail (21) that's 72 hits; with Fragile Strength (21 × 1.5 = 32), 47. Every number shows its sum, and the share link carries your whole build to paste on Discord.
You start with 3 and can reach 11. And with any free notch you can equip one charm too many and be overcharmed: you take double damage.
It depends on the boss. The calculator ranks the change in hits per notch against the enemy you choose, so the answer is the one for your fight.