How to limit ticket purchase to 1?

To do it, first click on the widget and then click on the wizard icon: 

Once you're on the wizard, you should find the option to set a max and min selection, there you should set up 1 as max selection.

Once you do that the widget will allow to select only one available ticket, other options will not be available. 

Here's my sample form: https://form.jotform.com/62364965575974 

Hope this helps.