Can I use Apple Pencil with iPhone X ?

No. See the following from the Compatibility section of Buy Apple Pencil - Apple

iPad Models

• iPad Pro 12.9-inch (2nd Generation)
• iPad Pro 12.9-inch (1st Generation)
• iPad Pro 10.5-inch
• iPad Pro 9.7-inch
• iPad (6th Generation)

There are 3rd party stylii that work with the iPhone and certain note taking Apps if that's what you want.

Adonit is highly regarded as a maker of compatible stylii.

The dash works with iPhones.

http://www.adonit.net/dash/

And they suggest Apps you may want to use with it:

http://www.adonit.net/jot-ready-apps/

This is a user-to-user support forum. No one here has information about unannounced products but you can make the suggestion to Apple via Feedback - iPhone - Apple