The number seven is the greater part of ten, its simplely represents many.
For instance the earth was made of manys days and on the last day God rested.
Yes 10 is a nice number
The number of days of creation at the start of the Old Testament is clearly an allegory for something else. We now know that this is not the way that the world as we know it came into being so the 7 day account is representative of some other meaning but significantly centred round the number 7.
Now 7 was also the sacred number of the much much older Sumerian civilisation so it would perhaps be wise to look there for the origins of this sacred number.
Moving much nearer our time the number 7 was the number used a great deal in the Book of Revelations. So is 7 a number of Revelation? and why?
The OT clearly also sees it as a sacred number because the 7th day was designated as Holy, a day when the works were completed