How to Increase Max Input Vars in WordPress using .htaccess file
The PHP Max Input Vars is the maximum number of variables your server can accept in a single request. The default value is 1000. Most modern WordPress themes require about 2000, so here we are.
Be sure to backup your site, or at least the .htaccess file, before you start!
Locate the .htaccess file at the root of your WordPress installation and click on edit.
Add the following line of code at the end of the file
php_value max_input_vars 3000
Note that some hosting providers may not grant you access to modify this file.