@echo off&setlocal
set "name1=Bart"
set "name2=Carol"
for /f "delims=" %%a in ('dir /b /a-d /o-d') do (
set "fname=%%~a"
set /a counter+=1
SETLOCAL ENABLEDELAYEDEXPANSION
call set "nname=%%name!counter!%%"
echo ren "!fname!" "!nname!%%~xa"
endlocal
)
Remove echo
to get it working.