Is it called iPhone "X" or iPhone 10?
The device is called iPhone X, pronounced iPhone 10.

The pronunciation follows on from (Mac) OS X being pronounced 10. In the special event, the announcement called the device out loud as ‘iPhone 10’. This is continued in the Apple video ‘Introducing iPhone X’.



This screenshot is showing the YouTube generated subtitles, as the manually written subtitles print ‘iPhone X’, which is what it's called but not how it's pronounced.