What is scale factor of iPhone X screen?

It's @3x. You can find the info in this video: https://developer.apple.com/videos/play/fall2017/801/

You might also want to checkout the Human Interface Guidelines for iPhone X (https://developer.apple.com/ios/human-interface-guidelines/overview/iphone-x/) which includes the following comment:

Supply high-resolution images for all artwork in your app. iPhone X has a high-resolution display with a scale factor of @3x. For glyphs and other flat, vector artwork, it's best to provide resolution-independent PDFs. For rasterized artwork, provide both @3x and @2x versions of your artwork. See Image Size and Resolution and Custom Icons.

Thank you!