I want to chang the default quantity box into a new display, it is quantity dropdown list with fixed number for customers so they have to select quantity from this list.
You can try Magento 2 Quantity Dropdown extension, which changes quantity box from input box into dropdown list as you want. Using this extension, customers only can choose from dropdown quantity list to add product to cart.
Hope it helps.
Have a look at our Magento 2 Quantity Dropdown extension by Meetanshi lets you enable quantity dropdown list for customers to select the quantity of items instead of quantity input box.