I can’t see what the OKCupid site offers so don’t know what its needs might be, however I would say if designed and coded well, a Phoenix app on a dedicated server will take quite a lot load - and in some cases, significantly more in comparison to many other languages or frameworks.
The great thing is that dedicated servers are far cheaper than ‘hosting in the cloud’, so if you outgrow one, it should not be prohibitive to add another.
I would say if budget is an issue, just go for it! If you outgrow a server you can always add more later - and you’ll be surprised by how well a single (relatively beefy) server performs.
I would however stress the importance of making sure you do off-server back ups, and to have two HDs set up in a raid array (so if one fails your app won’t go down).
