9
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Assuming 0 is allowed to be used as the first digit in the number (e.g. 00, 01, etc.), 100 two-digit numbers can be made using 0 through 9.
2
90000
120 combinations using each digit once per combination. There are 625 combinations if you can repeat the digits.
ten factorial = 10! = 3,628,800
If you want 4-digit numbers, there are 24 of them.