“Large” servers like Bittrex, Binance, Poloniex, etc do not run on a server. Instead they are built using different technologies and infrastructure that enables them to leverage large server farms. Their apps are usually built as containerized microservices and they do a lot of stuff to ensure high availability that are simply not possible with WordPress. You can certainly use a big server to improve performance and scalability of the Exchange, but unfortunately I do not have any concrete numbers for you. Roughly speaking, I do not think that with this type of technology you would be able to serve millions of people. This is why I added the warning.
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