Seven é o número mágico mais poderoso.
“Yes, sir,” said Riddle. “What I don’t understand, though — just out of curiosity — I mean, would one Horcrux be much use? Can you only split your soul once? Wouldn’t it be better, make you stronger, to have your soul in more pieces, I mean, for instance, isn’t seven the most powerfully magical number, wouldn’t seven — ?”
“Merlin’s beard, Tom!” yelped Slughorn. “Seven! Isn’t it bad enough to think of killing one person?"
-- HP and the Half-Blood Prince
Como esta resposta diz, sete é considerado um número mágico em lendas e folclore da vida real, mas aqui é a confirmação de que sete tem significado mágico no universo .
As propriedades mágicas do número sete foram descobertas pelo Arithmancer do século XIII.Wenlock first scribbled down her ground-breaking theorem on the magical properties of the number seven at breakfast one day apparently on the back of an envelope, using her usual invisible ink. She then proceeded to send her cousin a letter, using what she later believed to be the very envelope concealing the theorem.
-- Miranda Goshawk's Book of Spells
Isso também é confirmado na carta de boas-vindas da Hufflepuff em Pottermore :
Bridget Wenlock, the famous thirteenth-century Arithmancer who first discovered the magical properties of the number seven