There are 6 digits. Each digit can take ten different numbers except for the first digit since it cannot be zero.
So:
9 x 10 x 10 x 10 x 10 x 10
900000 numbers.
Another way of thinking about this is to just count up to 999,999. Obviously there are 999,999 different numbers here. But since our number has to have 6 digits in them, we have to delete 99,999 numbers. Thus there are 900,000 different numbers.
