how many combinations of 6 digit numbers are there?
There are one million of them (999999 + 1). If repetition is not allowed (which is probably what you are referring to in your last sentence), you can pick any of ten digits for the first number, any of the nine remaining for the second, and so forth. This is 10 times 9 times 8 times 7 times 6 times 5 or 151,200.