The iPhone 8 was the 2017 iPhone, in line with the 6/6S/7/8 (4 years).
iPhone X that year was a "one more thing" special for the iPhone 10th Anniversary, Cook called it the future of the iPhone.
Right now, we have a 4-year history of iPhones: 4/4S/5/5S (yes), 6/6S/7/8, and I expect it will be X/XS/11/11S, with 11S having 5G cellular.
Nobody really knows until we get close to September 2020; we could have the 11S, we could have the 11S + 12 (a newly designed Pro), or we could just have the entirely new 12 line.
Point is - people shouldn't assume that next year will be a redesign of new hardware, because nobody knows anything but rumors at this early date.
We had rumors for this year's iPhone 11: completely expecting reverse wireless charging and USB-C, which so many people were sure of up until a couple of days ahead of announcement. *cough*