Usually...
/skin/common_files/customer/main/buy_now.tpl
/skin/common_files/customer/main/product_details.tpl
/skin/common_files/customer/main/cart.tpl
It depends if you're using the quantity input field, or quantity select dropdown, but in either case you need to add a 'step' attribute and possibly some additional JS validation code on top.
https://www.w3schools.com/tags/att_input_step.asp
https://www.smarty.net/docs/en/language.function.section.tpl
Also depends whether you want to do it store-wide, or on just a selection of products. If the latter, then you'll probably need a custom product field.