How many social security numbers are available if the only restriction is that the number 000-00-0000 cannot be assigned?

Each digit can be 0-9 (10 numbers)

there are 9 numbers in a social security number

multiply 10 nine times:

10 x 10 x 10 x 10 x 10 x 10 x 10 x 10 x 10 = 1,000,000,000 (1 billion) possibilities.

the restriction is it can't be all zeros so subtract 1 from the total

1,000,000,000 - 1 = 999,999,999 combinations