global $wpdb;

$sql = "SELECT order_id FROM {$wpdb->prefix}wcfm_marketplace_orders";
$sql .= " WHERE 1=1";
$sql .= " AND vendor_id = %d";
$sql .= " GROUP BY order_id";
$number_of_orders = count( $wpdb->get_results( $wpdb->prepare( $sql, $supplier_id ) ) );

echo "(".$number_of_orders." Sales)";