As the simpliest way is to sum airflows of all GPUs and multiply it by 2. For example 1 GPU fans produce about 200 m3/h, you have 5 rigs of 8GPU each: 200 * 8 * 5 = 8000 m3/h. So you need one 700mm axial fan (efficiency 15-16k m3/h) for this config
I am not really OK with this statement. This seems to be really overkill.
The half of what you said makes sense.