You need to put a ## in the new name somewhere as the VBScript replaces ## with the random number:
strNewName = Replace(strNewName, "##", rndNumber)If you're using the script on multiple files then beware that it doesn't test whether there is already a file with the same number.
Because of that I have updated my Unique Numbers rename script to add a RandomNumbers option. That script will check that the chosen number isn't already in use. You can also use the PadTo setting at the top of the script to control the range of random numbers that are generated. Hopefully that solves all the problems!