How do i know how strong of a pump i need in a custom loop?

[whyteboi]

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[Supafly]

EK website first site after web search for what you asked. Has a configurator that I believe will help once you add your components to it.
https://www.ekwb.com/custom-loop-configurator/

[Spec_Ops_Ape]

Pumps based on the Xylem D5 design are the about the best you can get. They have a high flow rate with excellent head pressure. Flow rate is meaningless if there is not enough pressure to overcome the restrictions in a loop. They tend to cost more as well.

Next are the DDC pumps which typically have half the flow rate and head pressure of the D5 pumps but are cheaper. They are also smaller than the D5s. DDC pumps are suitable for small loops, CPU/GPU and a 240 or 360mm radiator.
I used to run 2 DDC pumps in a largish loop but split them halfway in the loop just to try and keep the head pressure up. Now I run a single D5, its also quieter than the DDC pumps.

Angled fittings and tubing size also contribute to pressure loss in a loop.

AquaComputer have designed their own pumps some of which are quite powerful.