https://www.cnet.com/news/apple-2018-iphones-could-be-called-iphone-x-plus-iphone-x2/

What do we call the phone that comes after the iPhone X? We've wondered that ourselves, especially since Apple effectively killed the iPhone 9 after bringing out the iPhone 8, iPhone 8 Plus and iPhone X this past September. 

Don't hold your breath for the iPhone 11, according to analysts from Guggenheim Securities (via AppleInsider), who said that Apple could keep the iPhone X branding for its most premium models, and refer to its other new iPhones as simply "iPhone" without any additional numbering.