GNU Mailman has an option to add subject_prefix to emails.

* "Mailman" in this post means not Mailman 3 but Mailman 2, because I use OpenBSD.

We can count emails of a mailing list with the command named config_list.

Create a file, for examle, named cfg.py, and write in it:

subject_prefix = '[list-label %04d] '

and config_list it for $LIST_NAME (replace this with your list's name):

# /usr/local/lib/mailman/bin/config_list -i cfg.py $LIST_NAME

It will bring emails with subjects like "[list-label 0001] an actual subject".