here's some text pulled directly from the moodledocs wiki.
"For very large sites, you are much better starting with a small pilot and gaining some experience and insight. A "what hardware do I need for 50,000 user?" style post in the forums is highly unlikely to get a useful answer."
Take a look through the posts here in the hardware and performance forum - there's a lot of good advice but you won't find a specific architecture or hardware specs as this will vary depending on a wide range of factors.