Answer
1,000,000,000 numbers can be formed.
Work Step by Step
If each x is a digit from 0 to 9, then for each digit, there are ten possibilities (repetition of digits is allowed), and the Social Security number has nine digits.
Using the Multiplication Rule of Counting (see page 298):
$10\times10\times10\times10\times10\times10\times10\times10\times10=10^{9}=1,000,000,000$.
