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I got what you mean but unfortunately we do not have a feature that populates quantity via condition.
This can only be achieved when you modify your form source code and inject custom script to perform such function.
Let us know if you're willing with using your form source code and if you need assistance with the custom script.
Sorry been ages since I have had time to look at this. I would def need assistance with such a code if you offer of help is still on.
Here is our guide on how to embed your form using the full source code:
Once you have your page setup, please let us know and we will be happy to help you with setting up the script to update the fields.
Thank you for using JotForm!