Mathematica 12, supported GPUs

Tl;dr Version 12 supports CUDA 10.1, which includes support for Turing Architecture.

To download the most updated version, even if you’ve just updated & are having trouble:

Needs["CUDALink`"]
CUDAResourcesInstall["<path_to_paclet>", Update->True]

As noted by @ilian, version 12 supports Turing chips/architecture. What this means is that GPUs with these chips will be supported. Turing chips are in all RTX cards. You only gain more and more access to computing resources as you go from 2060->2080. 2080->2080 Ti brings you a "true" Turing chip, with the RTX Titan having a "fully unlocked" Turing chip. All of these use Cuda 10, which is what is supported by Version 12 of Wolfram Language and Mathematica.

I will not discuss the Nvidia GPU Turing Architectures further, as that is outside of the scope of this forum. However, please see here for the source of this concrete commented notation of this compatibility:

"Neural nets in Mathematica 12 will use CUDA 10 and be compatible with your GPU."

-Sebastian Bodenstein, Wolfram Research