The number seven is used frequently in the Scriptures to signify completion. The number eight (one more than seven) sometimes represents an abundance.
أَمَّا ٱلرَّقْمُ ثَمَانِيَةٌ (‏ أَيْ أَكْثَرَ مِنْ سَبْعَةٍ بِرَقْمٍ وَاحِدٍ )‏ فَيُمَثِّلُ أَحْيَانًا وُجُودَ وَفْرَةٍ مِنْ شَيْءٍ مَا .‏