http://wccftech.com/iphone-x-short-supply-4-million-units/

The iPhone X supply at launch has always sparked a meaningful debate, with complications in production and components shortage will expect people to wait for weeks on to get their hands on the $999 flagship. According to a popular analyst, only 2-3 million iPhone X units will be available at launch but says that production difficulties will eventually be overcome in the future.

According to Kuo, the iPhone X FPCB for the antenna is the most probable cause of creating this supply shortage. Murata, one of the supply chain partners has been experiencing difficulties in meeting specification requirements for this device, as detailed in the note below via MacRumors.