HTML Multiselect Limit

You can use jQuery

  $("select").change(function () {
      if($("select option:selected").length > 3) {
          //your code here
      }
  });

You would do this via javascript on the client side, and then add a check on the server side as well in case the client has disabled javascript.

Here is some basic client side code to give you an idea:

<html>
    <body>
        <form onsubmit="validate()">
            <select multiple="multiple" id="choose" name="choose"> 
                <option value="1">Value 1</option> 
                <option value="2">Value 2</option> 
                <option value="3">Value 3</option> 
                <option value="4">Value 4</option> 
                <option value="5">Value 5</option> 
                <option value="6">Value 6</option> 
            </select><br>
            <input type="submit">
        </form>
        <script>
        function validate()
        {
            var selectChoose = document.getElementById('choose');
            var maxOptions = 2;
            var optionCount = 0;
            for (var i = 0; i < selectChoose.length; i++) {
                if (selectChoose[i].selected) {
                    optionCount++;
                    if (optionCount > maxOptions) {
                        alert("validation failed, not submitting")
                        return false;
                    }
                }
            }
            return true;
        }
        </script>
    </body>
</html>

HTML Multiselect Limit

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