A report by a Korean outlet has estimated the S9 day-one pre-order sales numbers to stand at 180,000, down 31% from the S8's 260,000 figure. It's about expected, though, as the S9 doesn't actually offer a substantial upgrade over its predecessor.
https://www.notebookcheck.net/The-first-day-of-Samsung-Galaxy-S9-sales-in-Korea-show-it-lagging-behind-the-S8.289028.0.html