You should custom a validation rule for the tagbox.
$.extend($.fn.validatebox.defaults.rules, {
taglength: {
validator: function(value,param){
value = $.trim(value).replace(/\,$/,'');
var len = $.trim(value).split(',').length;
return len >= param[0] && len <= param[1]
},
message: 'Please enter a value between {0} and {1}.'
}
})