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.
Any questions? Feel free to contact us by clicking the chat icon at the bottom right corner of this page or at support@digismoothie.com.