1 titan x does the job good so far.
Right, I've been making do with my two Titan Blacks.
In all seriousness though, the game runs fine on much lesser setups. It even ran pretty well on my new laptop that just uses Intel HD 5500 Graphics on an i5-5200U. ("Ran," since I only use Debian "Jessie"/KDE 'Linux' on it now, and Elite isn't on Linux.)
By the way, I use the Titan Blacks for much more demanding things than video games, like "winning" at BOINC, for example. My computer doesn't even really notice Elite running on top of everything else.
I hope they improve the graphics potentials of Elite, especially that annoying pop-in.