How many number combinations are possible with 0123?

16 if you include 0 starting

12 if not

That is not correct.

0123 is a 4-digit number = (n)

therefore n! = 4! = 1*2*3*4 = 24 (if 0 is included as I assume that is the question)

123 is a 3-digit number = (n)

Therefore n! = 3!= 1*2*3 = 6