To add to Peter’s comment, it is very much depending on the technology you use which in turn might depend on the software architecture.
If you can create a condition in which you can serve cached pages 99% of the time (by using caching plugins or varnish in front of your site) then you will be able to actually handle a lot of traffic with a single server of 8 or 16 GB of memory, but if every request requires access to a db then you might need same size of server just for the DB itself.
So basically, unless you just have a lot of money that you want to spend, you ask the wrong question and instead of looking for a server that will fit the requirements you should ask yourself how can you optimize your site in a way that will make it consume less resources.