This snippet allows you to limit the number of checkboxes that can be checked, and grays out and disables all other choices when that number is reached.

gr-limit-checkbox-selection.js

/**
* GravityRanger Limit Checkboxes
*
* This snippet allows you to limit the number of checkboxes that can be checked, and grays out and disables all other choices when that number is reached.
* 
* 
* @instructions:
* 
*  1. Change 2 on line 24 to the number of checkbox choices you want the user to be able to select.
*  2. Change the 1_5 on line 27 to the formID_fieldID of your checkbox field.
*  3. Add the scipt to an HTML field within your form or use a code snippet plugin.
* 
* @version  1.0
* @author   GravityRanger
* @license  GPL-2.0+
* @link     http://gravityranger.com/
*
*
*/


<script>
jQuery(document).ready(function($) {
    var maxCheckboxes = 2; // Change this to your desired maximum number of checkboxes.

    // Replace "input_1_5" with the ID of your checkbox field. You can find it in the Gravity Forms settings.
    var checkboxField = $('#input_1_5 input[type="checkbox"]');

    checkboxField.on('change', function() {
        var checkedCheckboxes = checkboxField.filter(':checked');

        if (checkedCheckboxes.length >= maxCheckboxes) {
            // Disable and grey out all other checkboxes.
            checkboxField.not(':checked').prop('disabled', true).closest('label').css('color', '#999');
        } else {
            // Re-enable all checkboxes when the limit is not reached.
            checkboxField.prop('disabled', false).closest('label').css('color', '');
        }
    });
});
</script>