Velidan,
<!DOCTYPE HTML>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css">
</style>
<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.11.0/jquery.min.js"></script>
<script>
$(function(){
var len = $("div:visible").length;
alert('Видимых блоков '+len);
});
</script>
</head>
<body> <div style="display: none;">1</div>
<div style="display: none;">2</div>
<div style="display: block;">3</div>
<div style="display: block;">4</div>
<div style="display: none;">5</div>
</body>
</html>