There is an error when you try to setup the "Show quantity picker" for most of the free themes; in particular, the "Brooklyn" theme. Under the "PRODUCT OPTIONS FORM" you have a "Picker type" where you can select "button" or "dropdown." The "dropdown" option does not work.
Also, when you select any of the options, whether it is "button" or "dropdown," the user is able to type the letter "e" into the form for "quantity." This should be a numbers based only form. I did not mess with any of the code or anything like that. I simply clicked the option to use it from Shopify, and noticed the issues when I was error checking the website. Please fix this issue. Thanks.
Could you kindly share your store URL so that everyone can take a look. Also, by adding a custom code you can restrict the input box to accept only numbers and not text.
If the quantity selector comes built-in with the theme it will work properly; try installing a fresh copy of the theme.
I'm afraid that unfortunately there is no simple copy-paste solution otherwise I'd happily guide you through.
In case you're unfamiliar with these technologies you'll likely want to hire a Shopify developer to assist you.