Limit Wordpress RSS Items using parameters

functions.php

<?php
add_action('pre_get_posts', function($query) {
    if ($query->is_feed() && filter_input(INPUT_GET, 'count', FILTER_VALIDATE_INT) && filter_input(INPUT_GET, 'count', FILTER_VALIDATE_INT)) {
        $query->set('posts_per_rss', filter_input(INPUT_GET, 'count', FILTER_VALIDATE_INT));
    }
    return $query;
});