What are the combinations for 1234567890?

First position can be any one of 10 numbers.
One number has been taken so the second position can be any one of 9 numbers.
Third position can be any one of 8 numbers.
7,6,5,4,3,2...
Last position can only be 1 number as all the others will have been taken.
Which leaves you with 10x9x8x7x6x5x4x3x2x1 or 10! (factorial).
10! = 3,628,800 combinations.

If numbers can be repeated this makes things easier as you can just multiply by 10 for each position - 10x10x10x10x10x10x10x10x10x10 or 10^10.
10^10 = 10,000,000,000 combinations.