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 .htaccess, php.ini , .user.ini or 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.

1. In your WordPress root installation locate any of these .htaccess, php.ini, .user.ini or wp-config.php.

2. Edit the file and paste the directive code.

3. Once you are done making changes save the file. If you have downloaded the file to make this change, upload it back to your server and override the existing file.

Directive for .htaccess
php_value max_input_vars 4000

Directive for php.ini or .user.ini
max_input_vars = 4000

Directive for wp-config.php
@ini_set('max_input_vars',4000);

The minimum recommended limit is 4000