Hi Sergio,

There is no fixed rule, but here are some tips:

If you don't want to render all the 15360x8640 pixels, then you could render only the Frustum and you will potentially need far fewer machines than otherwise: https://aximmetry.com/learn/tutorials/for-studio-operators/using-led-walls-for-virtual-production/#frustum-rendering

While rendering the Frustum, you can even render the rest of the Led Walls using reduced resolution Fill or fixed position Fill: https://aximmetry.com/learn/tutorials/for-studio-operators/using-led-walls-for-virtual-production/#fill-rendering 

If you want to render all the 15360x8640 pixels, that is 4 times more pixels than an 8k render. So if you can render your scene in a maximum of 8k without your machine being overloaded, then you would need 4 machines at least.

Also, it all depends on how your Led Walls can receive the video signal. Like how many inputs the Led Walls have and how many outputs your capture cards can provide.
And for example, if your render walls have 6 inputs, then you might want 3 machines where each machine supplies two of the inputs of the Led Walls. In this case, adding one more machine will likely not give any benefit for the setup as there would be still machines that will need to render for two Led Walls (if all the Led Wall inputs are the same resolution).

Warmest regards,