codice:
<body onload="sum()">
<form action="<?php echo $_SERVER['PHP_SELF']; ?>" method="POST">
<p id="demo2">Punti </p>
n1 <input type = "number" id = "n1"  min="0" max="5" value="0" onchange="sum()"/>
n2 <input type = "number" id = "n2"  min="0" max="5" value="0" onchange="sum()" />
n3 <input type = "number" id = "n3" min="0" max="5" value="0" onchange="sum()" />
n4 <input type = "number" id = "n4"  min="0" max="5" value="0" onchange="sum()" />
n5 <input type = "number" id = "n5"  min="0" max="5" value="0" onchange="sum()" />
<br>
<input type="submit" id="form" value="Allenati" disabled="disabled">
</form>


<script type="text/javascript">
function sum() 
{ 
    var tot, num1, num2, num3, num4, num5, dif, result; 
    tot = 5;
    num1 = parseInt(document.getElementById("n1").value, 10);
    num2 = parseInt(document.getElementById("n2").value, 10);
    num3 = parseInt(document.getElementById("n3").value, 10);
    num4 = parseInt(document.getElementById("n4").value, 10);
    num5 = parseInt(document.getElementById("n5").value, 10);
    dif = (num1+num2+num3+num4+num5);
    result =  (tot - dif);
    
    document.getElementById("demo2").innerHTML = result;
    
    if(result == 0)
    {
    document.getElementById("form").disabled = false;
    }
}
</script>


</body>
è un codice molto spartano ma a me basta che sia funzionale ^^