Tedrik,
id неповторимо
<!DOCTYPE HTML>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
</head>
<body>
<form name="9mmammo">
<input type="text" id="count1" value="1" oninput="calc1(+this.value||0)"><div>Сумма: <span id="result1">5</span> руб.</div>
</form>
<form name="556ammo">
<input type="text" id="count2" value="1" oninput="calc2(+this.value||0)"><div>Сумма: <span id="result2">10</span> руб.</div>
</form>
<script type="text/javascript">
function calc1(amount)
{
document.getElementById('result1').innerHTML = amount * 5;
}
;
function calc2(amount)
{
document.getElementById('result2').innerHTML = amount * 10;
}
</script>
</body>
</html>