place another if statement around the <form....> and </form> tag so that the checkout button wont do anything unless the qty is 10 or less:
find the form tag add the if statement:
<?php if($yourCartName->TotalColumn("Quantity") <= 10) { ?>
<form......>
<?php } ?>
same if statement for the closing </form> tag:
<?php if($yourCartName->TotalColumn("Quantity") <= 10) { ?>
</form>
<?php } ?>