max_input_vars: how many am I using?

Question

max_input_vars is the limit on the total number of GET, POST, and COOKIE vars. To determine how many there are you can just count them.

Here’s a quick example in a WordPress context:

function wpd_admin_error_notice() {
    echo 'This request contained ' . count( $_POST ) . ' POST vars, ' . count( $_GET ) . ' GET vars, and ' . count( $_COOKIE ) . ' Cookies.'; 
}
add_action( 'admin_notices', 'wpd_admin_error_notice' );

Of course, the above will not be entirely helpful for your purposes, because WordPress uses the standard post / redirect / get pattern for admin form submissions. You’ll have to hook the above code to an action that runs when your options are saved, and log them to a file or the db.