* "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
Create a file, for examle, named
cfg.py, and write in it:
subject_prefix = '[list-label %04d] '
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".