Let's take a look at a couple of data points. First, mobile engagement platform Localytics sent ZDNet information on the iPhone X market share after two weeks. The company's data shows the iPhone X market share to be at 1.42 percent. To put that into some context, after two weeks the iPhone 7 hit 2.7 percent. However, remember that the iPhone X is competing against iPhone 8 sales, so this skews the market.
It's also worth noting that the iPhone market has grown substantially since the iPhone 7 was released, distorting the figures a bit in the favor of the iPhone X.
Another data source we have is collated by Mixpanel, which shows iPhone X adoption rapidly catching up with iPhone 8. They have adoption figures of 1.75 percent and 1.95 percent, respectively, but they're lagging behind iPhone 8 Plus adoption, which, according to Mixpanel, is at 2.49 percent.
While it's hard to draw concrete conclusions from third-party data, the tentative conclusions I'm coming to is that the combination of iPhone 8 and iPhone 8 Plus and iPhone X are selling well, but what's hard to tell is how well they are selling compared to previous launches.
The fact that we've not had an Apple PR blitz telling us how awesome fast they are flying off the shelves suggests that the data may not be as good as Apple hoped it would be, and that the company is going wait until end of the quarter earnings to announce sales (by then it's likely that any supply shortages will have mostly worked themselves out).