Change the default number of related products

woo-single-product.php

/**
 * ----------------------------------------------------------------------------
 * CHANGE THE NUMBER OF RELATED PRODUCTS
 * ----------------------------------------------------------------------------
 */

add_filter( 'woocommerce_output_related_products_args', 'mw_related_products_args' );
function mw_related_products_args( $args ) {

    $args['posts_per_page'] = 4; // 4 related products
    $args['columns']        = 2; // arranged in 2 columns
    return $args;
}