Script to batch rename, have first file as a number, the rest sequential alphabetical

You really need a script for this. Is there a list of what files use what filenames? I have a rename script that I use in production, this is a modified version that will accept an input and rename selected files based on that. You could enumerate as many variants as you needed.

#target bridge

if(BridgeTalk.appName == 'bridge'){

    FT = MenuElement.create('command', 'Rename', 'at the end of Tools');

    FC = MenuElement.create('command', 'Rename', 'after Thumbnail/Open', this.menuID);

    }

FT.onSelect = function(){

    Renamer1();

    }

FC.onSelect = function(){

    Renamer1();

    }

function Renamer1(){

    app.synchronousMode = true;

    var thumbs = app.document.selections;

    var newName = '';

    var newNameU = '';

    var Title = [];

    var counter = 0;

    var variant = ['a', 'b', 'c', 'd'];

    try{

        newName = prompt('Enter part number');

        if(newName != null){

            newNameU = newName.toUpperCase();

            for (var a in thumbs){

                var selectedFile = thumbs.spec;

                Title = selectedFile.name;

                Title = (Title.split( '.' ))[0];

                thumbs.name = thumbs.name.toUpperCase();

                thumbs.name = newNameU + variant[(counter)] + '.jpg';

                counter++;

                thumbs.name = thumbs.name.toUpperCase();

                }

            }

        }

    catch(e){

        }

    app.document.refresh();

    app.synchronousMode = false;

    }