Since the value of one digit doesn't affect the value of the other digits, you can simply multiply the number of possibilities for each digit together. That is, if there are ten possible digits for each of nine positions, then that is
10 10 10 ... 10 (nine times).
For example, a three digit number can have 1000 possible values.
If you are actually thinking of Social Security numbers then it's not that simple. Look at
Hope this helps,
Stephen and Penny