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
max_input_vars
is an attempt for PHP to solve some security issues and when set, it limits your number of inputs (thus, fields in your forms). Also beware of
max_input_nesting_level
And yes - they are configurable. Just edit your php.ini or htaccess values.