Alter "posts_per_rss" option with querystring param to limit the number of posts returned for an RSS Feed

functions.php

<?php

/**
 * Alter "posts_per_rss" option with querystring param to limit the number of posts returned for an RSS Feed.
 */
add_filter( 'option_posts_per_rss', 'my_posts_per_rss' );
function my_posts_per_rss( $option ) {
    if( isset( $_GET['limit'] ) ) {
        return (int) $_GET['limit'] ?: $option;
    }
    return $option;
}