Yes.

No.

Yes, one D5 is enough.

I know it can be confusing when you get conflicting responses.

One D5 will provide sufficient flow to cool your proposed loop. Two pumps will not significantly lower temperatures on your hardware as opposed to a single pump. I have tried running my loop (2x420 rads, 3xGPU, CPU, RAM, Motherboard full cover, Aquaero block, Aqualis Reservoir, Aquacomputer filter) with one pump configuration as well as two pump configuration and I did not notice a significant difference in the performance of the loop.

Enjoy your build!

One will say yes and another will say no.

In any case, you will need at least one pump, and you will probably find that it is enough. But beyond the practically useless pump curves and graphs of so-called 'testing' websites you will decide what suits you regardless, in spite of the 'science'. If you want the real answers and want to eke out every last bit of heat from your loop you need to look up text books on fluid dynamics and thermodynamics, and apply your own data from your own loop to see what works.

Personally, I like the look of two pumps together on a dual pump top, even though I know that one is enough. It isn't about what I need but what I want and how I want my machine to look. I can run both my pumps at minimum and achieve 0.8 litres per minute flow rate, or I can boost them up to maximum at about 2.8 litres per minute and it makes bugger all difference to how my cooling loop performs.

Enjoy the responses!

I knew I could draw out my friend cc01 with a reply!

yongtw....your 3 480 AMS rads are incredibly restrictive radiators. if you are wanting maximum performance, you'll need another pump. if you don't care about getting the best performance out of your loop....i'm sure one pump will still move some water.

good luck, and enjoy your build!

Yeah the AMS rads are quite restrictive. I was running 3 slim rads with slightly higher levels of restriction plus one CPU block and a GPU full cover block and with a flow sensor I was measuring really quite low flow values of less than 3 LPM at full power with one D5.
With all those blocks as well you could be adding up to some serious flow rate drop.