There's an option to automatically set up the upsell product's quantity based on the quantity of the targeted or parent product. It's called "Match parent product's quantity" and basically, if a customer adds 5 of the parent product, then he/she will get 5 of the upsell product.
To set it up:
Go to the app dashboard and create/open an upsell offer.
Scroll down to Additional settings.
Under Quantity Options, put a checkmark on the Match parent product's quantity option.
Click the Save button.
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