Hi Hans-Peter,
Since the plugin isn’t configured to allow several fields for the messages to be shown (to be compatible with multilingual sites), I have a better suggestion for you, and it’s ever more convenient from a UX perspective.
Since you want to allow these 5 values only, and instead of typing that in the product description, you better use dropdown and not quantity input.
To enable this, go to Quantity Dropdown page, enable, and then go to Maximum Quantity page, enable it and enable per product as well there.
Next in the product page, you’ve already defined 1,2,3,6,9,12, just add maximum as 13, and you will have a dropdown 🙂
Let me know if this solves your issue.
Omar