In newer PHP-Versions the count of input-fileds per formula (POST) will be limited to 1000 (unverified information). It seams that this limit is already installed in certain
If you cannot/ do not want to increase the server limit, here is another solution
....
then using jquery
Using POST I tested posting 10000 records.
In the server
$cboxes = $_POST['cboxes']; $cbox_exp =(explode(',', $cboxes)); print_r(count($cbox_exp)); //gives me 10000