This is not working because the numbering is not successive. This whole configuration implementation is of course nothing to be proud of.

plugin { #setting_name = value

# archive folder mailbox_alias_old = Archive mailbox_alias_new = Archief mailbox_alias_old2 = Archive mailbox_alias_new2 = Archief mappen mailbox_alias_old3 = Archive mailbox_alias_new3 = Archived messages mailbox_alias_old4 = Archive mailbox_alias_new4 = Archived mail mailbox_alias_old5 = Archive mailbox_alias_new5 = Archives

# spam folder mailbox_alias_old10 = Spam mailbox_alias_new10 = Junk mailbox_alias_old11 = Spam mailbox_alias_new11 = Junk E-mail mailbox_alias_old12 = Spam mailbox_alias_new12 = Ongewenste e-mail

# drafst folder mailbox_alias_old20 = Drafts mailbox_alias_new20 = Concepten

}

https://wiki2.dovecot.org/Plugins/MailboxAlias