which CPU / Motherboard for multiple GPU?

I'm planning a build for productivity and machine learning / deep learning, hence not gaming. It must be Ryzen based (more cores/threads the better)

I am trying to understand which CPU/Motherboard/chipset should I choose which support 2xGPU, maybe 3.

I know with Threadripper there wouldn't be any problems, but the cost would be substantially higher.

Should I stay with Ryzen 9, (3900X or 3950), which motherboard?
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