Two month ago I wrote a long description how to update the current ticket counter, but I found a much better way to do it by command line.
The value of the last ticket number is stored in the table ticket_counters, but can be changed by commandline as well:
Display all counters:
Ticket::Counter.pluck(:id, :content)
Change counter:
Ticket::Counter.update(content: 123, id: 1)
I made a pull request for the documentation to add this, maybe someone could review it: Pull requests · zammad/zammad-documentation · GitHub