PHP Max Input Vars is the maximum number of variables your server can use for a single function to avoid overloads.
The max_input_vars directive can be set in
wp-config.php file. To gain access to these files you will need FTP credentials. If you do not have these please contact your hosting provider.
php_value max_input_vars 4000
@ini_set( 'max_input_vars' , 4000 );
The minimum recommended limit is 3000
Changing server directives is recommended to site owners who are confident to do so. If you rather have professionals do this for you, contact your hosting provider.