Why does our number system have only 10 different symbols? [duplicate]

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,

• When did $7$ days come in to being? but I think people had $10$ fingers and thumbs. Just a theory. I will google ;). – Chinny84 Jul 29 '15 at 16:16
• I once helped prepare gutenberg.org/ebooks/16449 – Hagen von Eitzen Jul 29 '15 at 16:16
• @Chinny84: There is nothing specific with 7. But what is the real significance of number 9. I think I should edit my question. – explorer Jul 29 '15 at 16:19
• It's somewhat unclear what you're asking. Base 10 is arbitrary; different cultures used different bases. The symbols "0" and "1" are also arbitrary symbols (they don't represent fingers) which are historically derived from the Hindu–Arabic numeral system. We don't have a unique digit for ten because otherwise we would be using base 11. – Insert Pseudonym Jul 29 '15 at 16:19
• @InsertPseudonym That book Hagen von Eitzen prepared shows pretty clearly that although different cultures use different bases, the bases are always related to 10; typically if they are not base 10 they are base 20, or an elaboration on base 5, or an elaboration on base 10 like the Babylonians' 60. They are never, for example, base 12 or 8. – MJD Jul 29 '15 at 16:30

If I show you both hands two times and then three fingers, I show you a total of $23$ fingers, where the $2$ and the $3$ excellently represent the two double-hands and the three fingers. Likewise, one double-hand and no extra fingers should therefore be represented as $10$, not with yet another digit.
Prime numbers are not really convenient as bases for number systems. There is rather an advantage in having divisors, which is likely why some other number systems where based around $60$ and $12$ is a also common (recall a dozen, and the timescale).
The reason for base $10$ being the ten fingers is plausible, the fact that it is a kind of convenient size and has some divisors might have contributed to it.