batch operation

[poster]

@fpefpe

music n

where n is 1 to 70 – each music folder contains the mp3 files – and
the number of mp3 vary from folder to folder

the covers are in a folder called “covers” and jpg’s are
numbered 1 to 70

So I understand you right that i.e. you want all mp3-files in the folder "music 1" to have the file 1.jpg in the folder "covers" embedded?

This should work:
Your Cover folder i.e. is d:\covers

Load all mp3-files from the mp3-folder including the subfolders to Mp3Tag.
Mark all files and define an action.

Action-type: Import Cover from file
Format string for image filename:
d:\covers$trim($right(%_directory%,2)).jpg