Recent posts in current category?

Here’s how I’d get the 20 (at most) most recent posts in the category.php page:

<?php
    $max_posts = 20;
    $cat_ID = get_cat_ID ( single_cat_title( '', false ) );
    $args = array(
        'category' => $cat_ID,
        'numberposts' => $max_posts,
    );
    $myposts = get_posts( $args );
    foreach( $myposts as $post ) {
        setup_postdata( $post );
    ?>
    <!-- add your HTML here, including the_title(), the_excerpt(), etc -->
    <?php

    }
?>

The code uses get_cat_ID() and single_cat_title() to get the ID of the current category. I’ve done some (very) minimal testing to make sure that it at least excludes posts that aren’t in the current category.