Forse cosi ( non so se esiste un metodo + semplice )
codice:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">

<html>
<head>
<title>Untitled</title>
<script language="JavaScript" type="text/javascript">
<!--
function conta() {
 c     = 0;
 oColl = document.getElementsByTagName('INPUT');
 for (i=0;i<oColl.length;i++) {
  curInp = oColl.item(i);
	if (curInp.type == 'text' && curInp.name.match(/^Campo[[]\d+[]]$/)) { 
	 c++;
	}
 }
 return c;
}
//-->
</script>

</head>
<body>
<form name="nomeMiaform">
<input type="text" name="Campo[0]"> 
<input type="text" name="Campo[1]"> 
<input type="text" name="Campo[2]">
<input type="text" name="Campo[122]">
<input type="text" name="Campo_[122]">
</form> 
<script language="JavaScript" type="text/javascript">
<!--
alert(conta())
//-->
</script>

</body>
</html>