This question already has an answer here:
- Harry Potter: Why 7? 6 answers
"...isn't seven the most powerfully magical number, wouldn't seven-?"
Well is it? The list link lacks...magic, most of the events being mundane or even coincidental.
Edit; because the question is fairly trivial without this change: Are there any examples of the number seven being magical in nature? In particular is there any actual magical advantage to having 7 horcruxes, or is it just sentiment?