Gravity Perks Limit Choices plugin allows you to limit how many times each choice on a multi-choice field may be selected (e.g. Radio Buttons, Checkboxes, Drop Downs, etc). It works with Product and Options fields with these input types as well. You need to install the Gravity Perks plugin in order to use this plugin.
Gravity Perks Limit Choices 1.7.3 GPL Latest Version