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According to Make line item quantities adjustable | Stripe Documentation,
Set adjustable_quantity on your line_items when creating a Checkout Session to enable your customers to update the quantity of an item during checkout.
Hope this helps! Let me know if there are any further questions or issues.
— @samliew
P.S.: Did you know, the concepts of about 70% of questions asked on this forum are already covered in the Make Academy. Investing some effort into it will save you lots of time and frustration using Make later!