You have been assigned a 9-digit identification number. how many identification numbers are possible?

Permutations are all possible variations, in which a set of numbers or characters can be ordered. The number of permutations on a set of  n elements is given by  n!. In our case we have 9 digits, which means 9 elements. 
There are 9!=9*8*7*6*5*4*3*2*1=362880 possible variations, or 362880 possible identification numbers.