You should use the `pre_get_posts` filter for this. For example, you could do this in your theme's functions.php file:

    function limit_category_posts( $query ) {
        if ( $query->is_archive() ) {

            // Do not do this for the date or category archives.
            if ( $query->is_date() || $query->is_category() )
                return $query;
            
            if ( is_category( '8' ) {
                $query->set( 'posts_per_page', 2 );
            }
        }
        return $query;
    }
    add_filter( 'pre_get_posts', 'limit_category_posts' );