The GNU coreutils mv already has an option specifying that you want to move to a directory: -t / --target-directory. If the argument to that option doesn't exists, mv will complain instead of moving all of your files to the same filename.
I would have written your mover as follows:
find . -name '* 9?.mp3' -exec mv -t 90 {} + Note the use of + instead of \;, globbing as many filenames together as possible, resulting in faster execution.