Should I use 2x16GB or 4x8GB RAM for a Ryzen 5000 build?

Go with 2x16GB. Two sticks are almost always better for Ryzen 5000 because the memory controller prefers fewer ranks per channel with less electrical load, making higher frequency and tighter timings easier to achieve. You also keep two slots open for a future 64GB upgrade, whereas 4x8GB locks you into 32GB forever. Only choose 4x8GB if you already have two 8GB sticks and buying two more is cheaper.