This script will prefix each selected track's Song Name with the number of its order in the selection (01, 02, 03...and so on). The selected tracks must be in a playlist that can be sorted by Play Order, that is, a user-created playlist, and not a library category playlist. The selection of tracks need not be contiguous but it is important to have already established the Play Order. Read Me explains more.
You may prefer the more modern Increment Number Tags.
"Number Song Names by Play Order" will prefix each selected track's Song name with the number of its order in the selection. In the example above, I have set the playlist to be sorted by Play Order and selected four tracks. Those four tracks' Song Names have been prefixed with the number of their sequential order in the selection (by virtue of the playlist being sorted by an established Play Order). Of course, you could select all the tracks in the playlist and they would be numbered accordingly as well.