Tafsir of Ash-Shu`ara' 26:190-191

"Indeed, in that is a sign, but most of them were not believers. And indeed, your Lord—He is the Exalted in Might, the Merciful."

This is the conclusion of the seven narratives that were presented for the reason you have previously learned. It has been said that the limitation to this specific number is because it is a complete number, though I entrust the knowledge of the secret behind that—and likewise the secret behind the arrangement of the stories in this specific order—to the Presence of the Knower of the Unseen, Glorified is His Majesty.