set the quantity filed name to contain the ID from the recordset:
<input name="quantity_<?php echo $row_RsName['ID']; ?>" id="quantity_<?php echo $row_RsName['ID']; ?>" type="text" value="1" />
then modify the quantity for the get contents from recordset to use the ID column to:
$cartNameCart_itemQuantity = floatval("".((isset($_POST["quantity_".$row_RsName['ID']."]))?$_POST["quantity".$row_RsName['ID']."]:"") ."");