Hi Henning,
If you are using WooCommerce, you can edit the ticket in WooCommerce (you’ll see the link to do this under the Advanced section of the ticket edit area), go to the Inventory tab, and check the “Sold individually” option.
Otherwise, you would need to add some custom code. We are limited in the amount of support that we can provide for customizations, but if you would like to go this route, let me know and I will try to point you in the right direction.
Thanks,
Jennifer