Apple's strategy for iPhone 20 depends a lot on what happens in 2027. Either Apple could release the iPhone 19 as expected, or it could skip to iPhone 20 as a twentieth anniversary device similar to iPhone X.
Not much is known at this point, and rumors are scarce. Apple could release an all-glass "iPhone XX" in 2027 alongside an iPhone Fold if rumors are true.
If Apple instead releases an iPhone 19 in 2027, then the iPhone 20 will release in 2028. There are no rumors about the 2028 device at this time.