How can I cap the number of tickets sold on jotform?

I'm trying to cap the number of tickets sold on this form from 50 to 100, could you let me know how to set this up? 

Thanks. 

We may use the user defined amount feature in Paypal to manually calculate the number of tickets purchased.

Guide:https://www.jotform.com/help/275-How-to-Pass-a-Calculation-to-a-Payment-Field

And use the Inventory widget to hold our ticket quantity.

To set this up:

1.)We add the Inventory Widget.

and define the available quantity. Do note to require the Inventory widget.

2.) We add the Form Calculation widget to calculate the quantity of ticket.

3.) We pass the value of the calculation into our Payment Integration.

When the Inventory widget gets 0 tickets, the form will not be submitted because the field is required.

Here is my sample form:https://form.jotform.com/82486792223969

Please give it a try and let us know how it goes.

Thank you.