Hank here from Shopify.
Depending on what theme you are using, may already include a quantity selector within, although it may not be customizable without the editing of the theme code.
If you are using a Shopify theme, the selector may be included within the editor (Online Store > Themes > Customize > Product Pages), you need to just enable it.
However, this will only enable the dropdown rather than allow you to edit the wording. For that, you may need some additional coding to implement this, instead of an app.
Lastly, if you share your store URL then hopefully, someone with more technical skills in the community will come across this and be able to add some value to you here, but if not, your best bet might be to reach out to one of our Shopify Experts, who you can hire to do almost any custom work on your store.
If you were to reach out to them about this query, they would be able to resolve this issue for you.
Let me know if you have any other questions!
All the best