This is something of an unfair question because the ISBN is currently 13 digits, but the last digit is a checksum digit calculated from all the others. So only 12 digits are independent, and since these 12 can each be any number from 0 to 9, there are 10^12 possible numbers: 1,000,000,000,000 from 000-0-000-00000-0 to 999-9-999-99999-4.