FFMPEG - Convert MP3 and JPEG to MP4, different JPEG to every MP3

I want to convert every MP3 with different JPEGs and output an MP4.

1. mp3 and 1.jpg = 1.mp4
2. mp3 and 2.jpg = 2.mp4
3. mp3 and 3.jpg = 3.mp4 etc

How can I make code to get this right?

I use a batchfile

@echo off
mkdir converted
for %%a in ("*.mp3") do ffmpeg  -i "%%a" -loop 1 -i image1.jpg -c:a copy -c:v libx264 -shortest "converted\%%~na.mp4"
pause

Something like this: Updated.....

@echo off

for %%a in (*.mp3) do call :CreateVideo "%%~a"

pause
exit

:CreateVideo
set /a Counter+=1
set CounterZero=000%Counter%
set CounterZero=%CounterZero:~-3%
ffmpeg -loop 1 -y -i "image%CounterZero%.jpg" -i "%~1" -shortest -acodec copy -vcodec libx264 "%~n1.mp4"
goto :EOF