woocommerce/docs/code-snippets/number-of-products-per-row.md

Change number of related products output

Add code to your child theme’s functions.php file or via a plugin that allows custom functions to be added, such as the Code snippets plugin. Avoid adding custom code directly to your parent theme’s functions.php file as this will be wiped entirely when you update the theme.

Please note that it does not work for all themes because of the way they’re coded.

if ( ! function_exists( 'YOUR_PREFIX_related_products_args' ) ) {
	/**
	 * Change number of related products output.
	 *
	 * @param  array $args The related products args.
	 * @return array The modified related products args.
	 */
	function YOUR_PREFIX_related_products_args( $args ) {
		if ( ! is_array( $args ) ) {
			$args = array();
		}

		$args['posts_per_page'] = 4; // 4 related products.
		$args['columns']        = 2; // Arranged in 2 columns.

		return $args;
	}
}
add_filter( 'woocommerce_output_related_products_args', 'YOUR_PREFIX_related_products_args', 20 );