Hi Carsten!
Thanks for reaching out to us! Let me help you with this topic.
We have a great article that will get you started on customizing our templates > https://theeventscalendar.com/knowledgebase/themers-guide/
For that specific template you are refering to, you should see the tickets.php template file located at /wp-content/plugins/event-tickets-plus/src/views/wootickets/
Bear in mind that the quantity input is actually a call to a WooCommerce function > woocommerce_quantity_input() which outputs the quantity-input.php template file located at/wp-content/plugins/woocommerce/templates/global/. So you might also see the WooCommerce documentation on how to customize their templates > https://docs.woocommerce.com/document/template-structure/
I hope that helps! Let me know if you have other questions and I’d be happy to help.
Best,
Victor