I'm trying to build a dual AMD Epyc 9654 for rendering, can I use all threads?

it depends on many many factors and in the price of just such a system it is irritating for just asking this question!

can you give more information about your studio / indie setup, what projects do you render? (simple CG animation or huge shots with simulations that are multiple Gigabytes per frame)

why only render? such a system can be used for massiv simulations.