The answer to this is contained within one line of rend_dxg.ini
TextureSetSize=428561408
At this point the question changes to asking what the largest possible value is that can be contained in this line.
Since a .ini file is a config file that consists of text-based content with a structure and syntax, which then contains key–value pairs for properties, then this then default to asking:
What is the largest possible text file?
I have heard of particularly knavish jackdaws inventing gigabyte and terabyte sized text files.
If you could build an even larger magellanic cloud of Deep Thought supernatural-computers capable of godzillaflops, running horrorbytes and gehennabytes sized text files, then my suspicion is that there is no upper bound to the number of triangles NR could render before either it crashes or the FPS is nonexistent.