Assuming you're not accepting answer like "00000024", and so you want an "actual" 8 digit number, the answer is
10 000 000
i.e. 1 followed by 7 zeroes, i.e. 10 millions. In fact, it is an even number (it ends with zero), and any number less than this has less than 8 digits:
10 000 000 - 1 = 9 999 999
which has 7 digits.