The reason we aren't seeing any 2018 iPhone X rumors is because it will be a marginal upgrade over the current iPhone X. 2019 is when the main refresh will be.
I think you’ll just see an iPhone 9 and iPhone 9 Plus which are basically upgraded 8’s and an iPhone 11 with a new model added an iPhone 11 Plus. It would be best for profitability to continue to add models slowly so the company can get repeat upgraders each year. That business model has worked well for the iPhone. Don’t give your customers everything they want at once.