check against database?

i have this html form

<form action="insert.php" method="post">
ko: <INPUT NAME="ko" SIZE=7 MAXLENGTH=20 onKeyPress="return numbersonly(this, event)">
<input type="submit" />
</form>

HTML:

Then i have this php code

<?
include "config.php";
?>

$sql="INSERT INTO bokning (ko)
VALUES
('$_POST[ko]')";

if (!mysql_query($sql,$con))
  {
  die('Error: ' . mysql_error());
  }
echo "X record added";

mysql_close($con)
?>

PHP:

let's say the limit for "ko" is set to 3 in the database(Table named "admin", collumn named "komax") and someone enters 5 into the form and presses submit, before it's enterd into the database, how can i make it sum up every row for "ko" in the table, "bokning" and give an error if it exceeds the limit of "komax" ?

 

Sorry, Can you describe in details. I cannot understand your problem.