codice:
<script language="JavaScript" type="text/javascript">
 function conta(val) {
    max=50
        if (form.elements[val].value.length > max) {
            form.elements[val].value = form.elements[val].value.substring(0,max)
        rest = 0
        } 
    else {
        rest = max - form.elements[val].value.length
        }
        form.num.value = rest
 }
</script>
codice:
<input type="text" name="campo" size="50" onKeyUp="conta(this.name)">
<input type="text" name="campodue" size="50" onKeyUp="conta(this.name)">
<input value="29" size="3" name="num" disabled>