To be specific, I am just curious as to why does our number system have 10 different digits?Just for an example, why did it not end at 7? (it is the greatest prime number before 10, there are 7 days of week). Or similarly any other digit  

If we assume the digits were derived from the number of fingers(including thumbs) on our hand, then why is it so that **0** represents nothing, **1** represents one finger, then for the 10th finger, we don't have a unique digit, but have to ADD **one** more to **nine**. 

EDIT: My question also aims to clear one more thing. If it is derived from the number of fingers, then why don't we have a special symbol for the 10th finger,