<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<script language="JavaScript">
<!--

function visualizza() {
alert(num);
}
-->
</script>
</head>

<body>

<form name="inserimento" id="inserimento" action="prova.php" method="post">
<input type="button" name="numero" id="numero" value="10" onclick="visualizza();">
<form>

</body>
<script type="text/javascript">
var num=document.getElementById('numero').value;
</script>
</html>