Change number of thumbnails per row on WooCommerce product gallery.

function.php

<?php
// Do NOT include the opening php tag.

/**
 * Change number of thumbnails per row on product gallery
 *
 * @author Nic Beltramelli
 *
 * @param array $wrapper_classes The number of thumbnails per row.
 * @return array The modified wrapper class.
 */
add_filter(
	'woocommerce_single_product_image_gallery_classes', function ( $wrapper_classes ) {

		$columns = 5; // Change this to n. Default is 4.
		$wrapper_classes[2] = 'woocommerce-product-gallery--columns-' . absint( $columns );
		
		return $wrapper_classes;

	}
);

woocommerce.scss

.woocommerce {

	div.product {

		/* Product Gallery */

		div.images {
      
			.woocommerce-product-gallery__image {

				&:nth-child(n+2) {
					width: 20%;
				}
			}

			.flex-control-thumbs {

				li {
					width: 20%;
				}
			}
		}
	}
}