Review count per product

WooCommerce provides a function for this. You should not need to query the comments and count them:

add_shortcode(
    'reviews_count',
    function() {
        // Make sure we're on a Product.
        if ( function_exists( 'is_product' ) && is_product() ) {
            // Get a WC_Product object for the current product.
            $product = wc_get_product( get_queried_object_id() );
            // Return the review count.
            return $product->get_review_count();
        }
    }
);

If you want to display the reviews count for a specific product you can use:

add_shortcode(
    'reviews_count',
    function() {
        if ( function_exists( 'wc_get_product' ) ) {
            // Get a WC_Product object for the product.
            $product = wc_get_product( 12345 );
            // Return the review count.
            return $product->get_review_count();
        }
    }
);

If you want to be able to pass the product ID to the shortcode (like this: [reviews_count id="12345"]), you can use:

add_shortcode(
    'reviews_count',
    function( $atts ) {
        // Make sure an ID was passed,
        if ( ! empty( $atts['id'] && function_exists( 'wc_get_product' ) ) {
            // Get a WC_Product object for the product.
            $product = wc_get_product( (int) $atts['id'] );
            // Return the review count.
            return $product->get_review_count();
        }
    }
);