KGI Securities analyst Ming-Chi Kuo estimates that Apple will launch three new iPhone models in 2018. Two of these will sport OLED displays, the 5.8" and 6.5" models. According to Kuo, all of Apple's iPhones in 2018 will have the same full-screen notched design used in the iPhone X.
Note that the image above, produced by KGI, is just a render. Kuo sees the 5.8" iPhone (X2?) using the same resolution as the iPhone X (1125x2436, 458 PPI). The 6.5" model will actually sport a higher density (480 to 500 PPI).