No, this is not what you don't study enough and your teacher kills you with a calculator. This would refer to a spell of ritual, which is designed to kill by things like the number of past targets, or a geometric arrangement of kill sites.
It's not just for killing enemies though, but also the key to break spells (though in this case, the other end usually involves massive death). For example, suppose you have an enemy that is impossible to kill because of various MacGuffin
sustaining its life. You could hunt down 6 or 7 of these, destroying them, in which case the enemy would suddenly be weak to your attacks.
A subtrope of Ritual Magic
and Geometric Magic
- Recurring theme in a witch story that the witch needs 5, 7, 12, 13, 100 or some such number sacrifices every now and then to preserve her eternal youth and beauty (it's always those last few meddling kids that give her trouble.
- This is done in Soul Eater, where they need 100 demons and one witch killed to become the Grim Reaper's weapon.
- Fullmetal Alchemist, the entire country is one big alchemy circle, where wars form each connection points.
- Harry Potter, the example mentioned above. Since it's a rare heroic version of this, death mainly comes in the form of destroyed objects rather than innocent people.
- This was done by the book in Dragon Quest VIII, where 7 sages needed to be killed (plus apparently, the statue in Neos needed to be smashed) to free Rhapthorne.