How can I limit the entries per date on submission?

I am creating an order form where the only days people can select are Friday and Saturday. I am trying to limit the amount of product that can be selected for each week.  

What I am trying to achieve is a total of 15 items per week. If there is a max of 15 already selected, then the person would need to select a different day to get their product. 

I don't care how far in advance people want to go or how many submissions they want to do, but what I do care about is how much can be ordered for each week. 

Does that make sense? 

What you're describing can be achieved using widgets that have predefined counters, which then decreases as users select an item/entry/slot. All the widgets I'm listing below track user selection for each submission. This would mean that when someone picks a slot/date/item/etc it will be counted towards the submission and the widget counter will reduce by 1. Feel free to check them out and pick one that will suit your needs.

1. Appointment Slots Widget

2. Date Reservation Widget

3. Inventory Widget

4. Ticket Purchases Widget

Related guide: How-to-Add-a-Widget-to-your-Form