max_input_vars define the number of variables that a server can take in a query via POST or GET method. Normally the value is set around 1000 by default. You can change this value using the following methods.
1. For Novice Users
The simplest way to increase WordPress’s Memory Limit is to ask your Hosting Provider to do it for you.
2. For Advanced Users
2.1 Edit php.ini file
If you have access to the php.ini file which can be easily found in the server file manager, change/add the following line in php.ini
max_input_vars = 15000; Maximum number of variables a server can parse
If you do not have access to php.ini file then you can add your own .user.ini file in the websites directory and add the code mentioned above into it. This is often considered safer since it does not affect the settings in phi.ini file.
2.2 Edit your .htaccess file
If you don’t have access to your php.ini file then add the following line to your .htaccess file.
php_value max_input_vars 15000