How do i limit the choices on a Checkbox field

You must be referring to your form http://www.jotform.us/form/52615648848165 Checkboxes field instead. Radio options button already are limited to single selection everytime.

So it has no limit property like the Checkboxes field.

The Checkbox field do have a Minimum and Maximum Selection option in its properties. 

See the screenshot below on how to setup the min/max selection option.

Hope this help. Let us know if you need further assistance.