Sorry, I am used to asking that question from doing so many years of sales. Storage is probably irrelevant. Without knowing how busy these 20 sites are I would venture to say a single socket server would be the best and most economical solution for you. We have customers with hundreds of sites on a single cpu. With that said, you can have 1 very popular website that may require numerous CPUs or numerous servers even. It’s all about how much usage these sites are going to get and what is running on those site.
All things being equal, we have seen amazing results out of the single CPU AMD EPYC. They benchmark as good as anything out there.