How many GPUs can you simultaneously output from in a single PC?

I’m aware that it’s technically possible to connect a large number of GPUs to a PC for mining purposes https://www.pcgamer.com/asus-has-a-motherboard-that-supports-up-to-19-gpus/, but how many of these can actually be used simultaneously for graphics output?

For example, if I wanted to use these all independently for a CAVE https://en.wikipedia.org/wiki/Cave_automatic_virtual_environment like system, where each GPU will render its own independent view to its own monitor. There should be very little interaction between GPUs. Is there some limitation of the OS, or graphics driver that would prevent me from doing this with more than something like four cards?

To be clear, I’m not interested in anything like SLI here, just using each GPU independently from a single application (think split screen rendering, but each player having their own monitor). I’m currently experimenting with this from Vulkan with two GPUs, but the graphics API isn’t important to me if that puts some limit on this. I’m just curious what the upper limit is for this kind of setup.

Here’s an older example of the kind of system I have in mind: https://www.jvrb.org/past-issues/9.2012/3511

Sir this is gokul from india i just like to know how graphics card are made and is there training for the development of graphics card