Change the number of related products in the single product page

functions.php

/**
 * Change the number of related products in the single product page.
 *
 * @param  array $args
 * @return array
 */
function wc_custom_related_products_number( $args ) {
    if ( isset( $args['posts_per_page'] ) ) {
        $args['posts_per_page'] = 8;
    }

    return $args;
}
add_filter( 'woocommerce_output_related_products_args', 'wc_custom_related_products_number' );