You can have multiple video walls, each with up to 16 cameras.
HOWEVER, you will need a lot of GPU power to display 16 or more cameras all at the same time. Don't skimp on the GPU.
I have used the Nvidia Quadro P1000 before with 16 camera video walls and it has been good (make sure you use the latest drivers). This card has four video outputs (so you can attach four monitors).
If you wanted to display 32 cameras simultaneously then I would look at GPUs that were twice as powerful as the P1000. It is the video decoding performance you care about.
You can also use rotating video walls, that display 16 cameras at a time, and then rotate onto the next set. This would be much kinder on the GPU. The P1000 will handle this nicely.
Actually, if it was me, upon reflection, I would use one computer with a Nvidia Quadro P1000 for every 16 cameras you want to display at the same time. I tend to use i7 CPUs with 16GB of RAM.