<!DOCTYPE HTML>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.12.0/jquery.min.js"></script>
<script>
$(function(){
var d = $("#number"), n = +$("#number").text(), c = $('input[type=checkbox]');
c.on('change', function () {
d.html(n + c.filter(":checked").length);
});
})
</script>
</head>
<body>
<div id="number">0</div>
<input type="checkbox" value="1" name="option1" />
<input type="checkbox" value="2" name="option2" />
<input type="checkbox" value="3" name="option3" />
<input type="checkbox" value="4" name="option4" />
<input type="checkbox" value="5" name="option5" />
<input type="checkbox" value="6" name="option6" />
</body>
</html>