User blog comment:IamJakuhoRaikoben/Chapter 476: Bid Farewell to the Mage Girls, Review./@comment-25111584-20160307210952/@comment-4401815-20160307214759

If you count Zeref, then there are 13 Spriggans, so maybe that has significance. (I mean, "7" always has significance, so why not? :P)