Can I limit the total quantity of ALL orders?

I want to set up an online booking form to order pre-picked berries.  But I need to be able to limit the total qty available to order each day. In other words, I want to have the app know that "John Smith" can't order 5 baskets because my total qty for the day is 200 and our current orders are at 197.

You can use this widget for your products.

https://widgets.jotform.com/widget/inventory

It's called Inventory Widget where you can define the product, set available quantity and the quantity range available to be picked by the users.

Demo: https://form.jotform.com/61471146729964

If you want to connect this widget to the payment field, you have to use form calculation widget to get the total amount and then pass the amount to the payment field.

Guides: 

Perform calculation on form: http://www.jotform.com/help/259-How-to-perform-calculation-in-the-form

Pass the calculation to the payment field: http://www.jotform.com/help/275-How-to-pass-a-calculation-to-a-payment-field

If you need further assistance, just let us know. We'd be happy to help.