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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.