With 2 X 480 , 1 X 360, 2 GPU blocks and 1 CPU block, I think you can barely achieve 1 GPM flow with a single D5 variant pump at max setting even if you use the best components in terms of flow restriction. (Look at Flow - Pressure graphs in Martin's liquid lab) Some people are satisfied at 1 GPM, but some aren't. So it's up to you.
Personally, I would add another one for more flow rate and reliability.
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